Cisco CCNA: key terms

Flashcards for this exam Cisco CCNA guide Class dates

• 10 Gigabit Ethernet (10GbE)Ethernet at 10 Gbps over copper, twinax or fiber, defined in 802.3ae (fiber) and 802.3an (copper).Foundational
• 1000BASE-LXGigabit Ethernet using 1310 nm long-wavelength light, reaching 5 km (or more) over single-mode fiber and 550 m over multimode.Intermediate
• 1000BASE-SXGigabit Ethernet over multimode fiber using 850 nm short-wavelength light, reaching about 220-550 m depending on fiber grade.Intermediate
• 1000BASE-TGigabit Ethernet over all four pairs of Cat 5e or better twisted-pair copper, up to 100 meters.Foundational
• 100BASE-TXFast Ethernet over two pairs of Cat 5 or better UTP at 100 Mbps, up to 100 meters.Foundational
• 1024-QAMModulation with 1,024 constellation points carrying 10 bits per symbol, introduced in 802.11ax.Intermediate
• 10BASE-TThe original twisted-pair Ethernet standard: 10 Mbps over two pairs of Cat 3 or better UTP, up to 100 meters.Foundational
• 10GBASE-LR10 Gbps Ethernet over single-mode fiber at 1310 nm with a reach of 10 km.Intermediate
• 10GBASE-SR10 Gbps Ethernet over multimode fiber using 850 nm light, reaching 300 m on OM3 and 400 m on OM4.Intermediate
• 10GBASE-T10 Gbps Ethernet over four-pair twisted-pair copper with RJ45 connectors, up to 100 m on Cat 6a.Intermediate
• 2.4 GHz BandThe 2.400-2.4835 GHz ISM band used by 802.11b/g/n/ax, with only three non-overlapping 20 MHz channels in North America.Foundational
• 256-QAMModulation with 256 constellation points carrying 8 bits per symbol, introduced to Wi-Fi by 802.11ac.Intermediate
• 5 GHz BandWi-Fi spectrum from about 5.150 to 5.895 GHz divided into U-NII sub-bands, offering many non-overlapping channels.Foundational
• 6 GHz BandThe 5.925-7.125 GHz band (up to 1,200 MHz) opened to Wi-Fi 6E and Wi-Fi 7 in the US in 2020.Foundational
• A RecordA DNS record that maps a host name to an IPv4 address.Foundational
• AAAA RecordA DNS record that maps a host name to an IPv6 address.Foundational
• Access CategoryOne of four WMM traffic queues: voice (AC_VO), video (AC_VI), best effort (AC_BE), and background (AC_BK).Intermediate
• Access Control List (ACL)Ordered list of rules attached to an object or interface that permits or denies access to specific subjects or traffic.Foundational
• Access LayerThe edge layer of a hierarchical network where end devices connect.Intermediate
• Access PortA switch port that carries traffic for a single data VLAN (plus optionally a voice VLAN) without tagging.Foundational
• Acknowledgment NumberThe TCP field indicating the next byte the receiver expects, implicitly acknowledging all prior bytes.Intermediate
• Address Resolution Protocol (ARP)The protocol (RFC 826) that resolves a known IPv4 address to the corresponding MAC address on the local network.Foundational
• Adjacent Channel Interference (ACI)Corruption caused by transmitters on overlapping or nearby channels.Intermediate
• Administrative DistanceA Cisco value (0-255) ranking the trustworthiness of route sources; the lowest AD wins when protocols offer the same prefix.Foundational
• All-Nodes Multicast AddressThe link-local multicast group ff02::1, which every IPv6 node joins and which functions like a link broadcast.Intermediate
• All-Routers Multicast AddressThe link-local multicast group ff02::2, which every IPv6 router joins.Intermediate
• Allowed VLAN ListThe set of VLANs permitted to traverse a trunk, all VLANs (1-4094) by default on Cisco.Foundational
• Alternate PortAn RSTP port role for a discarding port that provides a backup path to the root bridge via another switch.Intermediate
• AntennaA device that converts electrical RF signals into radiated electromagnetic waves and back again.Foundational
• Antenna GainThe increase in signal strength in a given direction achieved by focusing radiated energy.Foundational
• AnycastOne-to-nearest communication in which the same address is assigned to multiple nodes and routing delivers traffic to the closest one.Intermediate
• Application Layer (L7)OSI Layer 7 (and top layer of TCP/IP), which provides network services directly to user applications.Foundational
• Area Border Router (ABR)An OSPF router with interfaces in area 0 and at least one other area, which summarizes routes between them.Intermediate
• ARP CacheThe table a host or router keeps of recently resolved IPv4-to-MAC mappings.Foundational
• ARP CommandA command that displays and modifies the ARP cache on Windows, Linux and macOS.Foundational
• Assured Forwarding (AF)A DiffServ PHB group of four classes (AF1-AF4) each with three drop precedences (AFx1-AFx3).Intermediate
• AttenuationThe loss of signal strength as it travels through a medium, measured in decibels.Foundational
• Authentication, Authorization, and Accounting (AAA)Framework that verifies identity, determines permitted actions, and records what users do.Foundational
• Authoritative Name ServerA DNS server that holds the official records for a zone and answers queries for it with authority.Intermediate
• Auto-SummaryAutomatic summarization of routes to classful network boundaries when advertised across a different major network.Intermediate
• Automatic Medium-Dependent Interface Crossover (Auto-MDIX)A port feature that detects the cable type and swaps transmit and receive pairs automatically, so either straight-through or crossover cable works.Foundational
• Automatic Private IP Addressing (APIPA)A Windows feature that self-assigns an address from 169.254.0.0/16 when a DHCP server cannot be reached.Foundational
• AutonegotiationAn Ethernet mechanism through which connected devices exchange capabilities and select the highest common speed and duplex.Foundational
• Autonomous Access PointA standalone AP that contains all configuration, management, and control functions locally.Foundational
• Autonomous System (AS)A set of networks under one administrative control with a single routing policy, identified in BGP by an AS number.Foundational
• Autonomous System Boundary Router (ASBR)An OSPF router that redistributes external routes into OSPF, originating Type 5 or Type 7 LSAs.Intermediate
• Backbone CablingCabling that connects equipment rooms, telecommunications rooms and entrance facilities, within or between buildings.Foundational
• Backup Designated Router (BDR)The OSPF router that monitors the DR on a multi-access segment and takes over if the DR fails.Intermediate
• BandwidthThe maximum data rate a link or path can carry, expressed in bits per second.Foundational
• Basic Service Set (BSS)An AP and its associated client stations, the basic building block of an 802.11 WLAN.Foundational
• Basic Service Set Identifier (BSSID)The 48-bit MAC address that uniquely identifies a BSS, usually the AP radio's MAC per SSID.Intermediate
• Beacon FrameA management frame broadcast periodically by an AP to advertise its BSS and capabilities.Foundational
• Best EffortThe default QoS model where all packets are treated equally with no guarantees, typically first in first out.Foundational
• Bidirectional TransceiverA transceiver that sends and receives on a single fiber strand using two different wavelengths.Intermediate
• Binary-to-Decimal ConversionTranslating between base-2 and base-10 values, an essential skill for IP subnetting.Foundational
• Bits per Second (bps)The unit for data rate on networks, scaled as Kbps, Mbps, Gbps and Tbps using powers of 1000.Foundational
• Block SizeThe increment between consecutive subnet addresses in the interesting octet, equal to 256 minus the mask value in that octet.Foundational
• Border Gateway Protocol (BGP)The path-vector EGP (BGP-4) that exchanges reachability between autonomous systems and is the routing protocol of the Internet.Foundational
• BPDU GuardAn STP feature that errdisables a port if it receives any BPDU, protecting PortFast edge ports.Intermediate
• BridgeA Layer 2 device that connects network segments and forwards frames based on MAC addresses, separating collision domains.Foundational
• Bridge ID (BID)An 8-byte STP identifier made of a 2-byte priority (with extended system ID) and the switch MAC address.Foundational
• Bridge PriorityThe configurable portion of the STP bridge ID, 0-61440 in steps of 4096, default 32768.Foundational
• Bridge Protocol Data Unit (BPDU)The STP message switches exchange to elect the root bridge and compute the loop-free topology.Intermediate
• BroadcastOne-to-all communication in which a single frame or packet is delivered to every host on the local segment.Foundational
• Broadcast AddressThe last address in an IPv4 subnet, with all host bits set to 1, used to send a packet to all hosts on that subnet.Foundational
• Broadcast DomainThe set of devices that receive each other's Layer 2 broadcasts; bounded by routers or by VLANs.Foundational
• Broadcast MAC AddressThe all-ones MAC address ff:ff:ff:ff:ff:ff, which sends a frame to every device in the broadcast domain.Foundational
• BSS ColoringAn 802.11ax identifier (1-63) in the PHY header that distinguishes overlapping BSSs on the same channel.Intermediate
• Burned-In Address (BIA)The factory-assigned MAC address stored in a network interface's hardware.Foundational
• Bus TopologyA topology in which all devices share a single backbone cable terminated at both ends.Foundational
• Cable ModemA modem that provides broadband Internet over a cable TV provider's coaxial network using DOCSIS.Foundational
• Cable TesterA tool that verifies continuity and correct pinouts on copper cables, detecting opens, shorts and miswires.Foundational
• Canonical Name Record (CNAME)A DNS record that makes one name an alias of another canonical name.Foundational
• Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA)The 802.11 medium access method in which stations sense the medium and back off randomly before transmitting.Foundational
• Carrier Sense Multiple Access with Collision Detection (CSMA/CD)The media access method used by half-duplex Ethernet: listen before sending, detect collisions, and retry after a random backoff.Foundational
• Carrier-Grade NAT (CGNAT)Large-scale NAT performed by an ISP so many subscribers share a small pool of public IPv4 addresses.Intermediate
• Category 5e Cable (Cat 5e)Enhanced Category 5 UTP rated to 100 MHz, supporting 1000BASE-T and 2.5GBASE-T up to 100 meters.Foundational
• Category 6 Cable (Cat 6)UTP or STP rated to 250 MHz, supporting 1 Gbps to 100 m and 10GBASE-T up to about 55 m.Foundational
• Category 6A Cable (Cat 6a)Augmented Category 6 cable rated to 500 MHz, supporting 10GBASE-T to the full 100 meters.Foundational
• Category 8 Cable (Cat 8)Shielded twisted-pair cable rated to 2000 MHz, supporting 25GBASE-T and 40GBASE-T up to 30 meters.Intermediate
• Central SwitchingForwarding client data through a CAPWAP tunnel to the controller, which bridges it to the wired network.Intermediate
• Challenge-Handshake Authentication Protocol (CHAP)PPP authentication protocol using a three-way challenge-response with a hash of a shared secret.Intermediate
• Channel BondingCombining adjacent 20 MHz channels into 40, 80, 160, or 320 MHz channels for higher data rates.Intermediate
• Channel Reuse PlanAn assignment of channels to APs so neighbors use different channels and same-channel APs are separated.Intermediate
• Channel WidthThe amount of spectrum a Wi-Fi channel occupies: 20, 40, 80, 160, or 320 MHz.Foundational
• ChecksumA value calculated over data so the receiver can detect corruption in transit.Foundational
• Circuit SwitchingA method that reserves a dedicated communication path for the duration of a session.Foundational
• Cisco Catalyst 9800 Series Wireless Controller (C9800)Cisco's IOS XE-based wireless controller family, available as appliances, VM (9800-CL), and embedded options.Intermediate
• Cisco Catalyst CenterCisco's enterprise network controller and assurance platform, formerly Cisco DNA Center, with northbound Intent REST APIs.Intermediate
• Cisco CleanAirCisco AP hardware-based spectrum analysis that identifies and classifies RF interference sources.Intermediate
• Cisco Discovery Protocol (CDP)Cisco-proprietary Layer 2 protocol that advertises device identity, platform, addresses, VLAN and capabilities to directly connected neighbors.Foundational
• Cisco Express Forwarding (CEF)Cisco's default topology-based switching mechanism using a precomputed FIB and adjacency table for fast forwarding.Intermediate
• Cisco IOS XECisco's Linux-based modular operating system for enterprise routers and switches such as Catalyst 8000 and 9000, running IOS as a daemon.Foundational
• Class A AddressAn IPv4 address with first octet 1-126 (leading bit 0) and a default mask of 255.0.0.0 (/8).Foundational
• Class B AddressAn IPv4 address with first octet 128-191 (leading bits 10) and a default mask of 255.255.0.0 (/16).Foundational
• Class C AddressAn IPv4 address with first octet 192-223 (leading bits 110) and a default mask of 255.255.255.0 (/24).Foundational
• Class D AddressAn IPv4 address in the 224.0.0.0-239.255.255.255 range (224.0.0.0/4), reserved for multicast groups.Foundational
• Class of Service (CoS)The 3-bit Priority Code Point in the 802.1Q tag used for Layer 2 QoS marking (IEEE 802.1p).Foundational
• Class Selector (CS)DSCP values CS0-CS7 (multiples of 8) that preserve backward compatibility with 3-bit IP Precedence.Intermediate
• Class-Based Weighted Fair Queuing (CBWFQ)A queuing method that guarantees minimum bandwidth to user-defined traffic classes during congestion.Intermediate
• Classful AddressingThe original IPv4 scheme that divided the address space into fixed classes A through E based on the leading bits.Foundational
• Classification and MarkingIdentifying traffic (by ACL, NBAR, DSCP, interface) and writing a QoS value into the packet so later devices can act on it.Foundational
• Classless Inter-Domain Routing (CIDR)The addressing scheme (RFC 4632) that replaced classful addressing by allowing any prefix length and route aggregation.Foundational
• Client-Server ModelA model in which centralized servers provide resources and services to client devices.Foundational
• Co-Channel Interference (CCI)Performance loss when nearby BSSs on the same channel must defer to each other's transmissions.Intermediate
• Coaxial CableA cable with a central conductor, dielectric insulator, metallic shield and outer jacket, used for cable TV, cable Internet and legacy Ethernet.Foundational
• Collapsed CoreA two-tier design in which the core and distribution layers are combined in the same devices.Intermediate
• CollisionTwo devices transmitting on a shared medium at the same time, corrupting both signals.Foundational
• Collision DomainA network segment where frames can collide with one another; every switch port is its own collision domain.Foundational
• Committed Information Rate (CIR)The average rate a provider guarantees or a policer/shaper enforces, in bits per second.Foundational
• Community StringA shared password-like string used for access control in SNMPv1 and SNMPv2c, sent in clear text.Intermediate
• Conditional TrustA switch QoS setting that trusts endpoint markings only when a specific device, such as a Cisco IP phone, is detected via CDP.Intermediate
• Congestion ControlTCP algorithms that adjust the sending rate based on signs of network congestion such as packet loss or delay.Intermediate
• Connected RouteA route automatically installed for the subnet of an interface that is up/up and has an IP address.Foundational
• Connection-Oriented CommunicationCommunication in which a session is established before data transfer and delivery is tracked with acknowledgments.Foundational
• Connectionless CommunicationCommunication in which each message is sent independently with no session setup, acknowledgments or delivery guarantee.Foundational
• Console PortA serial management port on network devices that provides direct command-line access without network connectivity.Foundational
• Control and Provisioning of Wireless Access Points (CAPWAP)IETF tunneling protocol (RFC 5415) between lightweight APs and controllers, using UDP 5246 and 5247.Foundational
• Controller DiscoveryThe process by which a lightweight AP locates and joins a WLC.Intermediate
• ConvergenceThe state in which all routers have consistent, accurate routing information after a topology change.Foundational
• Core LayerThe backbone layer of a hierarchical network that provides fast, redundant transport between distribution blocks.Intermediate
• Count to InfinityA distance-vector routing loop problem where routers keep incrementing the metric of an unreachable route.Intermediate
• CRC ErrorAn interface error counted when a received frame's FCS does not match the computed CRC, indicating corruption.Foundational
• Crossover CableA twisted-pair cable with T568A on one end and T568B on the other, swapping transmit and receive pairs to connect like devices.Foundational
• CrosstalkInterference in which a signal on one wire pair couples onto an adjacent pair, degrading signal quality.Foundational
• Customer Premises Equipment (CPE)Equipment located at the customer's site that connects to the provider's network, such as modems, ONTs, routers and set-top boxes.Foundational
• Cyclic Redundancy Check (CRC)An error-detecting code based on polynomial division, used in Ethernet's FCS and in storage systems.Foundational
• Data Link Layer (L2)OSI Layer 2, which delivers frames between nodes on the same local network using physical (MAC) addresses.Foundational
• Data Over Cable Service Interface Specification (DOCSIS)The CableLabs standard for delivering broadband data over cable TV coaxial networks.Intermediate
• Data RateThe PHY signaling speed at which bits are transmitted over the air, not user throughput.Foundational
• DatagramA self-contained, independently routed unit of data sent without a prior connection, commonly the UDP PDU.Foundational
• DecapsulationThe reverse of encapsulation, in which each layer on the receiver removes and processes its header before passing data up.Foundational
• Decibel (dB)A logarithmic unit expressing the ratio between two power levels; dBm expresses absolute power relative to 1 milliwatt.Intermediate
• Decibel Relative to Isotropic Radiator (dBi)Antenna gain in decibels compared with a theoretical isotropic radiator.Foundational
• Decibel-Milliwatt (dBm)Absolute power level in decibels relative to 1 milliwatt (0 dBm = 1 mW).Foundational
• Default GatewayThe router address a host sends traffic to when the destination is on a different subnet.Foundational
• Default RouteThe route 0.0.0.0/0 (or ::/0 in IPv6) that matches any destination not covered by a more specific route.Intermediate
• Default-Information OriginateA command that makes OSPF (or other protocols) advertise a default route into the routing domain.Intermediate
• Demarcation PointThe point where the service provider's network responsibility ends and the customer's begins.Foundational
• Demilitarized Zone (DMZ)A network segment between an organization's internal network and the Internet that hosts public-facing services.Foundational
• Designated Port (DP)The port on each segment that offers the lowest cost path to the root and forwards toward that segment.Foundational
• Designated Router (DR)The OSPF router elected on a multi-access segment to form full adjacencies with all others and originate the Type 2 LSA.Intermediate
• Destination UnreachableICMP type 3 message indicating a packet could not be delivered, with codes giving the reason.Intermediate
• DHCP Exclusion RangeAddresses within a scope that the DHCP server will not assign, reserved for statically configured devices.Foundational
• DHCP LeaseThe period during which a client may use an address assigned by DHCP.Foundational
• DHCP Option 150A Cisco-specific DHCP option that gives IP phones a list of TFTP server addresses for downloading configuration files.Foundational
• DHCP Option 43A vendor-specific DHCP option used to give APs the IP address of their wireless controller.Intermediate
• DHCP OptionsAdditional configuration parameters a DHCP server sends to clients, identified by number.Intermediate
• DHCP Relay AgentA router or Layer 3 switch function that forwards DHCP broadcasts from clients to a DHCP server on another subnet as unicast.Intermediate
• DHCP ReservationA DHCP configuration that always assigns the same IP address to a specific client, identified by its MAC address or client ID.Foundational
• DHCP ScopeThe range of IP addresses and associated settings a DHCP server can assign on a particular subnet.Foundational
• DHCPv6The IPv6 version of DHCP, which can assign addresses (stateful) or just options such as DNS servers (stateless).Intermediate
• Differentiated Services (DiffServ)A scalable QoS model that classifies and marks packets with DSCP values and applies per-hop behaviors at each router.Foundational
• Differentiated Services Code Point (DSCP)A 6-bit value in the IP header used to mark packets for quality-of-service treatment.Intermediate
• Diffie-Hellman (DH)Key agreement protocol that lets two parties derive a shared secret over an insecure channel.Foundational
• Diffusing Update Algorithm (DUAL)The EIGRP algorithm that guarantees loop-free paths and selects successors and feasible successors.Intermediate
• Digital Subscriber Line (DSL)Broadband Internet over telephone copper pairs, using frequencies above voice so phone service can share the line.Foundational
• Direct Attach Copper (DAC)A fixed-length twinax cable with transceiver connectors (such as SFP+ or QSFP28) permanently attached on both ends.Intermediate
• Directed BroadcastA packet sent to the broadcast address of a specific remote subnet, which a router could forward to that subnet.Intermediate
• Distance-Vector Routing ProtocolA routing protocol class where routers share their routing tables with neighbors and know only distance and direction to destinations.Foundational
• Distribution LayerThe aggregation layer of a hierarchical network that connects access switches and enforces policy, routing and summarization.Intermediate
• DNS CacheTemporary storage of DNS answers on clients and resolvers for the duration of each record's TTL.Foundational
• DNS over HTTPS (DoH)Encrypting DNS queries inside HTTPS on TCP 443 to provide privacy from on-path observers.Intermediate
• DNS over TLS (DoT)Encrypting DNS queries with TLS on dedicated TCP port 853.Intermediate
• DNS ZoneA portion of the DNS namespace managed as a unit, with its records stored in a zone file or directory.Intermediate
• DNS64A DNS function that synthesizes AAAA records from A records for IPv6-only clients, pointing them at a NAT64 prefix.Advanced
• Documentation Address RangeIP ranges reserved for examples in documentation and training: 192.0.2.0/24, 198.51.100.0/24 and 203.0.113.0/24 (TEST-NET-1 to 3).Intermediate
• Domain Information Groper (dig)A Linux/macOS DNS query tool that shows detailed responses, including flags, TTLs and authority sections.Intermediate
• Domain Name System (DNS)The hierarchical, distributed system that translates domain names into IP addresses and other records, using UDP and TCP port 53.Foundational
• Domain Name System Security Extensions (DNSSEC)DNS extensions that digitally sign records so resolvers can verify their authenticity and integrity.Intermediate
• DORA ProcessThe four-message DHCP exchange: Discover, Offer, Request, Acknowledge.Foundational
• Dotted-Decimal NotationThe format for writing IPv4 addresses as four decimal numbers (0-255) separated by periods.Foundational
• Dual StackRunning IPv4 and IPv6 simultaneously on the same devices and network.Foundational
• Dual-Band Access PointAn AP with radios operating simultaneously in two bands, typically 2.4 GHz and 5 GHz.Foundational
• Duplex MismatchA misconfiguration where one end of a link runs full duplex and the other half duplex, causing poor performance.Foundational
• Duplicate Address Detection (DAD)An IPv6 process that checks whether an address is already in use on the link before assigning it.Intermediate
• Dynamic Frequency Selection (DFS)A requirement that 5 GHz devices detect radar and vacate the channel when radar is present.Intermediate
• Dynamic Host Configuration Protocol (DHCP)A protocol that automatically assigns IP addresses and other configuration to hosts, using UDP ports 67 (server) and 68 (client).Foundational
• Dynamic IP AddressAn IP address automatically assigned to a device, usually by DHCP, for a limited lease period.Foundational
• Dynamic NATNAT that maps inside addresses to public addresses drawn from a pool on a first-come, first-served, one-to-one basis.Foundational
• Dynamic Trunking Protocol (DTP)A Cisco protocol that negotiates whether a link becomes a trunk, using modes dynamic auto, dynamic desirable, trunk and access.Intermediate
• East-West TrafficTraffic that moves laterally between servers, workloads or devices within a data center or network.Intermediate
• Effective Isotropic Radiated Power (EIRP)The highest signal strength radiated from an antenna: transmitter power minus cable loss plus antenna gain.Foundational
• EIGRP Composite MetricEIGRP's metric calculated from bandwidth and delay by default, with optional load and reliability via K values.Intermediate
• EIGRP Topology TableThe EIGRP table storing all learned routes with their FD and RD, including successors and feasible successors.Intermediate
• Electromagnetic Interference (EMI)Unwanted electrical noise from external sources, such as motors, fluorescent lights and power lines, that disrupts signals on copper cabling.Foundational
• Embedded Wireless Controller (EWC)Catalyst 9800 controller software running on a Catalyst access point or switch for small deployments.Advanced
• Encapsulating Security Payload (ESP)IPsec protocol (IP protocol 50) that provides confidentiality, integrity, authentication, and anti-replay.Intermediate
• EncapsulationThe process of each layer wrapping data from the layer above with its own header (and sometimes trailer) as it moves down the stack.Foundational
• Enhanced Interior Gateway Routing Protocol (EIGRP)Cisco-developed advanced distance-vector IGP using DUAL for loop-free fast convergence; published as informational RFC 7868.Foundational
• Ephemeral PortsShort-lived client-side source ports assigned by the operating system for outgoing connections, typically 49152-65535.Foundational
• Equal-Cost Multipath (ECMP)Installing and load sharing across multiple routes to the same prefix that have identical cost.Intermediate
• ErrdisableA Cisco port state where the switch shuts an interface due to a detected error such as BPDU guard, port security or UDLD.Intermediate
• EtherChannelCisco term for bundling multiple physical Ethernet links into one logical link for bandwidth and redundancy.Foundational
• EthernetThe dominant wired LAN technology, standardized as IEEE 802.3, defining framing, MAC addressing and physical media from 10 Mbps to 800 Gbps.Foundational
• Ethernet FrameThe Layer 2 unit Ethernet uses, containing destination MAC, source MAC, EtherType or length, payload and FCS.Foundational
• Ethernet Naming ConventionThe IEEE format rate-BASE-medium (for example 1000BASE-T) that describes an Ethernet physical layer.Foundational
• EtherTypeA 2-byte Ethernet II field identifying the protocol carried in the frame's payload.Intermediate
• Expedited Forwarding (EF)A DiffServ PHB (DSCP 46) for low-loss, low-latency, low-jitter traffic such as voice, usually served by a priority queue.Foundational
• Extended Service Set (ESS)Multiple BSSs sharing the same SSID and connected by a distribution system, allowing roaming.Foundational
• Extended System IDThe 12-bit field in the STP bridge priority that carries the VLAN ID, allowing a unique bridge ID per VLAN with one MAC.Intermediate
• External BGP (eBGP)A BGP session between routers in different autonomous systems.Foundational
• Feasible Distance (FD)The lowest total EIGRP metric from the local router to a destination, through the successor.Intermediate
• Feasible Successor (FS)An EIGRP backup neighbor whose reported distance is lower than the current feasible distance, guaranteeing a loop-free path.Intermediate
• Fiber Optic CableCable that transmits data as pulses of light through glass or plastic fibers, offering high bandwidth, long distance and immunity to EMI.Foundational
• Fiber to the Premises (FTTP)Broadband access in which optical fiber runs all the way to the home or business, also called FTTH for homes.Foundational
• File Transfer Protocol (FTP)A protocol for transferring files that uses TCP port 21 for control and port 20 (active mode) or a negotiated port (passive mode) for data.Foundational
• FirewallA device or software that permits or denies traffic between networks or hosts according to a security policy.Foundational
• First Hop Redundancy Protocol (FHRP)A class of protocols that provide a redundant default gateway through a shared virtual IP and MAC address.Foundational
• First In First Out (FIFO)The simplest queuing method where packets are transmitted in arrival order with no prioritization.Foundational
• FlexConnectCisco AP mode for branch sites that can switch client traffic locally and survive WAN outages.Intermediate
• Floating Static RouteA static route given a higher administrative distance than the primary route so it installs only when the primary fails.Foundational
• Flow ControlMechanisms that prevent a fast sender from overwhelming a slow receiver.Intermediate
• FragmentationSplitting an IP packet into smaller pieces so it can cross a link with a smaller MTU, then reassembling it at the destination.Intermediate
• FrameThe Layer 2 PDU, consisting of a header with MAC addresses, a payload, and usually a trailer for error checking.Foundational
• Frame AggregationCombining multiple frames into a single transmission to reduce overhead.Intermediate
• Frame Check Sequence (FCS)The 4-byte Ethernet trailer containing a CRC-32 value used to detect frame corruption.Foundational
• Free Space Path Loss (FSPL)Natural decrease in signal amplitude as an RF wave spreads out over distance.Foundational
• FrequencyThe number of times a wave completes a cycle per second, measured in hertz (Hz).Foundational
• Full MeshA topology in which every node has a direct link to every other node.Intermediate
• Full-DuplexCommunication in which a device sends and receives at the same time over separate paths, eliminating collisions.Foundational
• Fully Qualified Domain Name (FQDN)A complete domain name that specifies a host's exact location in the DNS hierarchy, including all labels up to the root.Foundational
• Fully Specified Static RouteA static route that names both the exit interface and the next-hop IP address.Intermediate
• Gateway Load Balancing Protocol (GLBP)Cisco-proprietary FHRP that provides gateway redundancy and load balancing using one virtual IP and multiple virtual MACs.Intermediate
• Generic Routing Encapsulation (GRE)A Cisco-developed tunneling protocol (IP protocol 47) that encapsulates almost any network-layer protocol in IP.Intermediate
• GiantAn Ethernet frame larger than the maximum allowed size on an interface, normally 1518 bytes untagged.Intermediate
• Gigabit Ethernet (GbE)Ethernet at 1000 Mbps, defined in 802.3z (fiber) and 802.3ab (copper).Foundational
• Global Routing PrefixThe leading portion of an IPv6 global unicast address assigned by an ISP or RIR to identify a site.Intermediate
• Global Unicast Address (GUA)A globally routable IPv6 address, currently allocated from 2000::/3.Foundational
• Gratuitous ARP (GARP)An unsolicited ARP message in which a host announces its own IP-to-MAC mapping.Intermediate
• Half-DuplexCommunication in which a device can either send or receive at any moment, but not both at once.Foundational
• HeaderControl information a protocol adds in front of the payload, such as addresses, ports, sequence numbers and flags.Foundational
• Hello PacketA periodic routing protocol message used to discover neighbors, verify parameters and maintain adjacencies.Foundational
• Hidden NodeA station that cannot hear another station's transmissions, causing both to transmit and collide at the AP.Intermediate
• Hold-Down TimerA timer during which a router ignores updates with worse metrics for a route recently marked unreachable.Intermediate
• Hop LimitThe 8-bit IPv6 header field, equivalent to the IPv4 TTL, decremented by each router and discarded at zero.Foundational
• Horizontal CablingCabling that runs from the telecommunications room patch panel to work area outlets on the same floor.Foundational
• Host PortionThe trailing bits of an IP address that identify a specific interface within a subnet.Foundational
• Hosts FileA local text file that maps host names to IP addresses and is checked before DNS on most systems.Foundational
• Hot Standby Router Protocol (HSRP)Cisco-proprietary FHRP with one active and one standby router sharing a virtual IP and MAC address.Foundational
• HSRP Interface TrackingReducing an HSRP router's priority when a tracked interface or object fails so the standby router can take over.Intermediate
• HSRP PreemptionAn HSRP option that lets a router with higher priority take over as active when it comes online.Foundational
• HubA legacy Layer 1 device that repeats incoming signals out every other port, creating a single shared collision domain.Foundational
• Hub-and-Spoke TopologyA WAN topology in which branch sites (spokes) connect to a central site (hub), and traffic between spokes passes through the hub.Foundational
• Hypertext Transfer Protocol (HTTP)The Application-layer request-response protocol for the web, using TCP port 80 by default.Foundational
• Hypertext Transfer Protocol Secure (HTTPS)HTTP carried inside TLS encryption, using TCP port 443 by default (and UDP 443 for HTTP/3).Foundational
• ICMP Echo ReplyICMP type 0 message returned in response to an echo request, confirming reachability and providing round-trip time.Foundational
• ICMP Echo RequestICMP type 8 message sent by ping to test whether a destination is reachable.Foundational
• ICMPv6The IPv6 version of ICMP (next header 58), which handles errors, diagnostics, Neighbor Discovery and multicast listener discovery.Intermediate
• IEEE 802.11The IEEE family of standards defining wireless LAN PHY and MAC layers, commonly known as Wi-Fi.Foundational
• IEEE 802.11a1999 amendment defining OFDM in the 5 GHz band with data rates up to 54 Mbps.Foundational
• IEEE 802.11ac2013 amendment (Wi-Fi 5) for 5 GHz only, adding 80/160 MHz channels, 256-QAM, up to 8 streams, and downlink MU-MIMO.Foundational
• IEEE 802.11axAmendment (Wi-Fi 6/6E) focused on efficiency in dense environments with OFDMA, UL/DL MU-MIMO, 1024-QAM, and TWT.Foundational
• IEEE 802.11b1999 amendment defining HR-DSSS in 2.4 GHz with data rates of 1, 2, 5.5, and 11 Mbps.Foundational
• IEEE 802.11beAmendment marketed as Wi-Fi 7, adding 320 MHz channels, 4096-QAM, Multi-Link Operation, and Multi-RU.Foundational
• IEEE 802.11eAmendment adding quality of service to the 802.11 MAC, including EDCA and HCCA.Intermediate
• IEEE 802.11g2003 amendment bringing OFDM (ERP) to 2.4 GHz with rates up to 54 Mbps, backward compatible with 802.11b.Foundational
• IEEE 802.11n2009 amendment (Wi-Fi 4) adding MIMO, 40 MHz channels, and frame aggregation in 2.4 and 5 GHz, up to 600 Mbps.Foundational
• IEEE 802.1QThe IEEE standard for VLAN tagging that inserts a 4-byte tag with a 12-bit VLAN ID into Ethernet frames.Foundational
• IEEE 802.1XPort-based network access control standard that authenticates devices before allowing traffic on wired or wireless networks.Foundational
• IEEE 802.3The IEEE working group and family of standards that define wired Ethernet physical layers and MAC.Foundational
• IGMP SnoopingA switch feature that listens to IGMP messages to forward multicast frames only to ports with interested receivers.Intermediate
• Implicit DenyDefault rule that blocks any traffic or access not explicitly permitted.Foundational
• Inside Global AddressIn Cisco NAT terminology, the public address that represents an inside host to the outside world.Intermediate
• Inside Local AddressIn Cisco NAT terminology, the address of an inside host as seen on the inside network, usually a private address.Intermediate
• Institute of Electrical and Electronics Engineers (IEEE)A professional organization whose standards association publishes the 802 family of LAN and MAN standards.Foundational
• Integrated Services (IntServ)A QoS model that reserves resources per flow end to end using RSVP signaling.Intermediate
• Inter-VLAN RoutingRouting packets between VLANs using a router or Layer 3 switch.Foundational
• Interface Configuration Command (ifconfig)A legacy Unix/Linux command for viewing and configuring network interfaces, replaced on Linux by the ip command.Foundational
• Interface ErrorsCounters on network interfaces that track problems such as CRC errors, runts, giants, collisions, drops and discards.Foundational
• Interface Identifier (IID)The host portion of an IPv6 address, normally the last 64 bits.Intermediate
• Interior Gateway Protocol (IGP)A routing protocol used to exchange routes within a single autonomous system, such as OSPF, IS-IS, EIGRP or RIP.Foundational
• Intermediate Distribution Frame (IDF)A secondary telecommunications room that serves a floor or area, connecting horizontal cabling to access switches and backbone links to the MDF.Foundational
• Internet Assigned Numbers Authority (IANA)The function, operated by PTI under ICANN, that coordinates global IP address allocation, DNS root zone management and protocol number registries.Foundational
• Internet Control Message Protocol (ICMP)The Layer 3 protocol (IP protocol 1) used for error reporting and diagnostics, such as ping and traceroute.Foundational
• Internet Engineering Task Force (IETF)The open standards body that develops and publishes Internet protocol standards as RFCs.Foundational
• Internet Group Management Protocol (IGMP)The IPv4 protocol hosts use to join and leave multicast groups with their local router (IP protocol 2).Intermediate
• Internet Key Exchange (IKE)Protocol that negotiates IPsec security associations and keys, using UDP 500 and UDP 4500 for NAT traversal.Advanced
• Internet LayerThe TCP/IP model layer that handles logical addressing and routing, equivalent to OSI Layer 3.Foundational
• Internet Message Access Protocol (IMAP)An email retrieval protocol that keeps messages on the server and synchronizes them across clients, using TCP 143 (or 993 with TLS).Foundational
• Internet Protocol (IP)The connectionless Layer 3 protocol that addresses and routes packets across interconnected networks.Foundational
• Internet Protocol Security (IPsec)Suite of protocols that authenticates and encrypts IP packets, used mainly for site-to-site and remote-access VPNs.Intermediate
• Internet Protocol version 4 (IPv4)The fourth version of IP, using 32-bit addresses written in dotted-decimal notation, defined in RFC 791.Foundational
• Internet Protocol version 6 (IPv6)The successor to IPv4, using 128-bit addresses written in hexadecimal, defined in RFC 8200.Foundational
• Internet Service Provider (ISP)A company that provides Internet access and related services to customers.Foundational
• Internet Small Computer Systems Interface (iSCSI)A protocol that carries SCSI block storage commands over TCP/IP, using TCP port 3260.Intermediate
• IP AddressA logical Layer 3 address that identifies a network interface and its network for routing purposes.Foundational
• IP Address ConflictA condition in which two devices on the same network use the same IP address, causing intermittent connectivity for both.Foundational
• IP Address Management (IPAM)Software and processes for planning, tracking and managing IP address space together with DHCP and DNS.Intermediate
• IP CommandThe Linux iproute2 utility for managing addresses, links, routes and neighbors.Foundational
• IP Configuration Command (ipconfig)The Windows command that displays and manages IP configuration.Foundational
• IP MulticastOne-to-many or many-to-many delivery where a source sends a single stream to a group address and routers replicate it only where receivers exist.Foundational
• IP Phone Boot ProcessThe startup sequence in which a Cisco IP phone gets power, learns its voice VLAN, obtains an IP address and TFTP server, downloads its config and registers.Foundational
• IP PrecedenceThe legacy 3-bit priority field in the IPv4 ToS byte, values 0-7, superseded by DSCP.Foundational
• IP Protocol NumberThe value in the IPv4 Protocol field (or IPv6 Next Header) that identifies the encapsulated upper-layer protocol.Intermediate
• iPerfAn open-source tool that measures network throughput between a client and server using TCP or UDP streams.Intermediate
• IPv4 HeaderThe 20- to 60-byte header at the front of every IPv4 packet carrying addressing and control fields.Intermediate
• IPv6 Address CompressionRules for shortening IPv6 addresses: omit leading zeros in each hextet and replace one longest run of all-zero hextets with ::.Foundational
• IPv6 Address NotationIPv6 addresses are written as eight groups of four hexadecimal digits separated by colons, with a /prefix length.Foundational
• IPv6 Documentation PrefixThe prefix 2001:db8::/32, reserved by RFC 3849 for examples in documentation.Intermediate
• IPv6 HeaderThe fixed 40-byte header of an IPv6 packet, with eight fields and optional extension headers chained after it.Intermediate
• IPv6 Link-Local Address (LLA)An IPv6 address in fe80::/10, automatically configured on every IPv6 interface and valid only on the local link.Foundational
• IPv6 Loopback AddressThe IPv6 address ::1/128, used by a host to send packets to itself.Foundational
• IPv6 Multicast AddressIPv6 addresses in ff00::/8 that identify groups of interfaces, replacing the role of IPv4 broadcast.Intermediate
• IPv6 SubnettingDividing an IPv6 prefix into smaller prefixes, normally on nibble boundaries and almost always ending with /64 LANs.Intermediate
• IPv6 Unspecified AddressThe all-zeros IPv6 address ::/128, used as a source address before a host has an address.Intermediate
• Iterative DNS QueryA DNS query in which the server returns the best answer it has, often a referral to another server, rather than resolving fully.Intermediate
• JitterThe variation in packet delay over time, which disrupts real-time traffic such as voice and video.Foundational
• Jumbo FrameAn Ethernet frame with a payload larger than 1500 bytes, commonly up to 9000 bytes.Intermediate
• KerberosTicket-based network authentication protocol using a trusted KDC and symmetric cryptography.Intermediate
• Late CollisionA collision detected after the first 64 bytes of a frame have been transmitted.Intermediate
• LatencyThe time it takes for data to travel from source to destination, usually measured in milliseconds.Foundational
• Leased LineA dedicated, private telecommunications circuit between two locations, rented from a carrier with guaranteed bandwidth.Foundational
• Lightweight Access Point (LAP)An AP that handles real-time radio functions while a controller manages configuration and control.Foundational
• Lightweight Directory Access Protocol (LDAP)A protocol for querying and modifying directory services such as Active Directory, using TCP/UDP port 389.Foundational
• Limited BroadcastThe address 255.255.255.255, which reaches all hosts on the local segment and is never forwarded by routers.Intermediate
• Link Aggregation Control Protocol (LACP)IEEE 802.3ad/802.1AX protocol that negotiates link aggregation bundles; modes active and passive.Foundational
• Link BudgetAn accounting of all gains and losses from transmitter to receiver to predict received signal strength.Intermediate
• Link LayerThe lowest layer of the TCP/IP model (also called Network Access or Network Interface) covering OSI Layers 1 and 2.Foundational
• Link Layer Discovery Protocol (LLDP)IEEE 802.1AB vendor-neutral Layer 2 neighbor discovery protocol using TLVs.Foundational
• Link-Local AddressAn address valid only on the local network segment and never forwarded by routers.Intermediate
• Link-State Advertisement (LSA)An OSPF data unit describing a router's links, a network, or external and summary prefixes, flooded to build the LSDB.Intermediate
• Link-State Routing ProtocolA routing protocol class where every router floods topology information and independently computes shortest paths with SPF.Foundational
• Load BalancerA device or service that distributes client requests across multiple servers to improve performance and availability.Foundational
• Load Balancing HashThe algorithm that selects which member link of an EtherChannel or ECMP group carries a flow, based on header fields.Intermediate
• Local Area Network (LAN)A network covering a limited area such as a home, office or building, usually owned and managed by one organization.Foundational
• Local ModeDefault Cisco AP mode that serves clients and tunnels all client traffic to the WLC through CAPWAP.Intermediate
• Logical TopologyThe way data flows through a network and how devices communicate, independent of physical cabling.Foundational
• Login BannerWarning message displayed before or at sign-in stating that use is monitored and unauthorized access is prohibited.Foundational
• Longest Prefix Match (LPM)The forwarding rule that the route with the most specific (longest) matching prefix is used, regardless of AD or metric.Foundational
• Loopback AddressAn address that refers to the local host itself; 127.0.0.0/8 in IPv4 (usually 127.0.0.1) and ::1 in IPv6.Foundational
• Loopback InterfaceA logical, always-up router interface used for management, router IDs and stable peering addresses.Foundational
• Loopback PlugA connector that routes a port's transmit pins back to its receive pins to test the port itself without a network.Foundational
• Low Latency Queuing (LLQ)CBWFQ with a strict priority queue for real-time traffic, policed during congestion to prevent starvation of other classes.Intermediate
• Lucent Connector (LC)A small form-factor fiber connector with a 1.25 mm ferrule and push-pull latch, usually used as a duplex pair.Foundational
• MAC AddressA 48-bit Layer 2 address that identifies a network interface on a local network, written as 12 hexadecimal digits.Foundational
• MAC Address TableThe switch table mapping learned source MAC addresses to ports and VLANs, used for frame forwarding (CAM table).Foundational
• Mail Exchanger Record (MX)A DNS record that identifies the mail servers that accept email for a domain, with preference values.Foundational
• Main Distribution Frame (MDF)The primary telecommunications room where the building's backbone cabling converges and where carrier circuits typically connect.Foundational
• Management FrameAn 802.11 frame type (type 0) used to join, leave, and maintain a BSS.Foundational
• Management Information Base (MIB)A hierarchical database definition describing the objects an SNMP agent exposes, such as interface counters and CPU usage.Intermediate
• Maximum Segment Size (MSS)The largest amount of TCP payload data a host will accept in one segment, advertised in the SYN.Intermediate
• Maximum Transmission Unit (MTU)The largest Layer 3 packet, in bytes, that a link can carry without fragmentation; 1500 bytes on standard Ethernet.Foundational
• Media Access Control Security (MACsec)IEEE 802.1AE standard providing Layer 2 hop-by-hop encryption and integrity on Ethernet links.Advanced
• Media ConverterA Layer 1 device that converts signals between different media types, such as copper twisted pair to fiber.Foundational
• Medium Access Control (MAC)The 802.11 sublayer of Layer 2 that handles addressing, framing, medium access, and security.Foundational
• Meraki DashboardCisco Meraki's cloud management interface for wireless, switching, security, and cameras.Foundational
• Mesh TopologyA topology in which devices have multiple interconnections, providing redundant paths.Foundational
• MetricA protocol-specific value expressing the cost of a path; within one routing protocol the lowest metric route is preferred.Foundational
• Metro EthernetA carrier service that delivers Ethernet connectivity between sites across a metropolitan area or wider WAN.Intermediate
• Metropolitan Area Network (MAN)A network spanning a city or metropolitan area, larger than a campus but smaller than a WAN.Foundational
• ModemA device that modulates digital data onto an analog carrier and demodulates it back, connecting a customer network to a provider medium.Foundational
• Modified EUI-64A method that builds a 64-bit IPv6 interface ID from a 48-bit MAC address by inserting fffe in the middle and flipping the seventh bit.Intermediate
• Modular QoS CLI (MQC)Cisco's three-step QoS configuration framework: class-map, policy-map and service-policy.Intermediate
• Modulation and Coding Scheme (MCS)An index combining modulation type and coding rate that determines a PHY data rate.Intermediate
• Monitor ModeAn AP mode that does not serve clients but scans all channels for rogues, intrusion, RRM, and location.Intermediate
• Multi-Fiber Push On (MPO)A rectangular multi-fiber connector holding 8, 12, 16, 24 or more fibers, used for parallel optics and high-density trunks.Intermediate
• Multi-Link Operation (MLO)Wi-Fi 7 feature allowing a device to use multiple bands or channels simultaneously as one logical link.Intermediate
• Multi-User MIMO (MU-MIMO)MIMO in which an AP sends different spatial streams to multiple clients simultaneously.Intermediate
• MulticastOne-to-many communication delivered only to hosts that have joined a particular group.Foundational
• Multicast Address RangesKey IPv4 multicast blocks: 224.0.0.0/24 link-local, 232.0.0.0/8 SSM, 233.0.0.0/8 GLOP and 239.0.0.0/8 administratively scoped.Foundational
• Multicast DNS (mDNS)A zero-configuration protocol that resolves .local names on the local link using multicast queries to UDP port 5353.Intermediate
• Multicast MAC AddressA MAC address with the least significant bit of the first byte set to 1, used to deliver frames to a group of interfaces.Intermediate
• Multigigabit Ethernet (NBASE-T)2.5GBASE-T and 5GBASE-T Ethernet (802.3bz), which deliver more than 1 Gbps over existing Cat 5e and Cat 6 cabling.Intermediate
• Multilayer SwitchA switch that can also perform Layer 3 routing (and sometimes higher-layer functions) in hardware.Foundational
• Multimode Fiber (MMF)Fiber with a larger core (50 or 62.5 microns) that carries multiple light paths, used for shorter distances inside buildings and data centers.Foundational
• Multiple-Input Multiple-Output (MIMO)Use of multiple transmit and receive antennas and radio chains to improve throughput and reliability.Foundational
• MultiplexingCombining multiple signals or data streams over a single shared medium.Intermediate
• Name Server Lookup (nslookup)A command-line tool on Windows, Linux and macOS for querying DNS servers.Foundational
• Name Server Record (NS)A DNS record that lists the authoritative name servers for a zone.Foundational
• NAT Traversal (NAT-T)Techniques that let protocols which embed addresses or lack ports, such as IPsec and VoIP, work through NAT devices.Intermediate
• NAT64A translation mechanism that lets IPv6-only clients reach IPv4-only servers by mapping IPv4 addresses into an IPv6 prefix.Advanced
• Native VLANThe VLAN whose frames are sent untagged across an 802.1Q trunk, VLAN 1 by default on Cisco.Foundational
• Neighbor Advertisement (NA)An ICMPv6 type 136 message replying to a Neighbor Solicitation with the sender's link-layer address, or announcing a change.Intermediate
• Neighbor Discovery Protocol (NDP)The ICMPv6-based protocol (RFC 4861) that performs address resolution, router discovery, prefix discovery and duplicate address detection in IPv6.Intermediate
• Neighbor Solicitation (NS)An ICMPv6 type 135 message used to find a neighbor's MAC address, verify reachability, or detect duplicate addresses.Intermediate
• NetBIOSA legacy API and set of services for name resolution and sessions on Windows networks, using UDP 137, UDP 138 and TCP 139.Intermediate
• NetFlowA Cisco-originated technology that exports metadata about traffic flows, such as addresses, ports and byte counts, to a collector.Intermediate
• Network AddressThe first address in a subnet, with all host bits set to 0, which identifies the subnet itself.Foundational
• Network Address Translation (NAT)A function that rewrites IP addresses (and often ports) in packet headers as traffic passes through a router or firewall.Foundational
• Network BaselineA record of normal network performance and behavior used to identify anomalies.Foundational
• Network File System (NFS)A file-sharing protocol common in Unix and Linux environments, using TCP and UDP port 2049.Intermediate
• Network Interface Card (NIC)The hardware that connects a device to a network, providing a physical connection and a MAC address.Foundational
• Network Layer (L3)OSI Layer 3, which provides logical addressing and routing of packets between different networks.Foundational
• Network PortionThe leading bits of an IP address, identified by the subnet mask, that specify which network or subnet the address belongs to.Foundational
• Network ProgrammabilityUsing software, APIs, and data models to configure, monitor, and operate networks programmatically instead of by CLI.Foundational
• Network SegmentationDividing a network into separate zones, such as VLANs or subnets, with controlled traffic between them.Foundational
• Network Statistics Command (netstat)A command that shows active connections, listening ports, routing tables and interface statistics.Foundational
• Network Time Protocol (NTP)A protocol that synchronizes clocks across network devices and hosts, using UDP port 123.Foundational
• Network TopologyThe arrangement of nodes and links in a network, described physically (cabling) or logically (data flow).Foundational
• Network Troubleshooting MethodologyThe structured seven-step process: identify the problem, establish a theory, test it, plan, implement, verify functionality and document.Foundational
• Network-Attached Storage (NAS)A storage device connected to a network that provides file-level access using protocols such as SMB and NFS.Foundational
• NIC TeamingCombining multiple NICs on a host into one logical interface for redundancy, load balancing or both.Intermediate
• NmapA widely used open-source network scanner for host discovery, port scanning, service detection and OS fingerprinting.Intermediate
• Noise FloorThe ambient level of background RF energy on a channel, measured in dBm.Foundational
• Non-Overlapping ChannelsChannels whose spectrum does not overlap, such as 1, 6, and 11 in the 2.4 GHz US band.Foundational
• North-South TrafficTraffic that enters or leaves a data center or network, moving between internal systems and external clients.Intermediate
• Northbound and Southbound APIsController interfaces: northbound APIs face applications and users, while southbound APIs face network devices.Foundational
• NTP StratumThe distance of an NTP server from an authoritative reference clock, from stratum 0 (the clock) to 15; stratum 16 means unsynchronized.Intermediate
• Null0 InterfaceA virtual Cisco interface that discards traffic routed to it, used for summary and blackhole routes.Intermediate
• Number of Subnets FormulaThe calculation 2^s, where s is the number of borrowed subnet bits, giving how many subnets are created.Foundational
• Object Identifier (OID)A dotted numeric identifier that uniquely names an object in the SNMP MIB tree.Intermediate
• OctetA group of 8 bits; an IPv4 address contains four octets.Foundational
• Omnidirectional AntennaAn antenna that radiates equally in all horizontal directions in a doughnut-shaped pattern.Foundational
• Open Shortest Path First (OSPF)An open-standard link-state IGP that uses areas, LSAs and Dijkstra's SPF algorithm with a cost metric.Foundational
• Open Systems Interconnection Model (OSI model)A seven-layer ISO reference model that describes how data moves from an application on one host to an application on another.Foundational
• Optical Network Terminal (ONT)The customer-premises device that terminates a fiber-to-the-premises connection and converts optical signals to Ethernet.Intermediate
• Optical Power Meter (OPM)A tool that measures optical power in a fiber, used with a light source to measure end-to-end loss.Intermediate
• Optical Time-Domain Reflectometer (OTDR)An instrument that injects light pulses into a fiber and analyzes backscatter and reflections to locate breaks, splices, bends and connector losses.Intermediate
• Organizationally Unique Identifier (OUI)The first 24 bits of a MAC address, assigned by the IEEE to identify the device manufacturer.Foundational
• Orthogonal Frequency-Division Multiple Access (OFDMA)Multi-user OFDM that divides a channel into resource units so several stations transmit simultaneously.Intermediate
• Orthogonal Frequency-Division Multiplexing (OFDM)Multicarrier modulation splitting a channel into many orthogonal subcarriers transmitted in parallel.Foundational
• OSPF AreaA logical grouping of OSPF routers and links that share an LSDB, limiting flooding and SPF scope.Foundational
• OSPF Area 0The OSPF backbone area to which all other areas must connect and through which all inter-area traffic passes.Foundational
• OSPF CostThe OSPF interface metric, calculated as reference bandwidth divided by interface bandwidth, summed along a path.Foundational
• OSPF Hello and Dead IntervalsOSPF timers: how often hellos are sent and how long to wait without hellos before declaring a neighbor down.Foundational
• OSPF Neighbor StatesThe progression an OSPF adjacency follows: Down, Attempt, Init, 2-Way, ExStart, Exchange, Loading, Full.Intermediate
• OSPF Network StatementAn OSPF configuration command that uses an address and wildcard mask to select interfaces and assign them to an area.Foundational
• OSPF Network TypesOSPF interface modes - broadcast, non-broadcast, point-to-point, point-to-multipoint and loopback - controlling DR election, timers and neighbor discovery.Intermediate
• OSPF PriorityAn interface value from 0 to 255 (default 1) used in DR/BDR election; 0 makes the router ineligible.Foundational
• OSPF Stub AreaAn OSPF area that blocks Type 4 and Type 5 external LSAs and receives a default route from the ABR instead.Intermediate
• OSPFv3The version of OSPF for IPv6 (and, with address families, IPv4) defined in RFC 5340.Intermediate
• Out-of-Band Management (OOB)Managing network devices through a dedicated path separate from production data traffic.Intermediate
• Outside Global AddressIn Cisco NAT terminology, the real, publicly routable address of a host on the outside network.Intermediate
• PacketThe Layer 3 PDU, consisting of an IP header plus a transport-layer payload.Foundational
• Packet Capture (pcap)The recording of network packets for analysis, and the file format (pcap/pcapng) that stores them.Foundational
• Packet Internet Groper (ping)A command that tests IP reachability and round-trip time by sending ICMP Echo Requests and waiting for Echo Replies.Foundational
• Packet LossThe percentage of packets that fail to reach their destination.Foundational
• Packet SwitchingA method in which data is divided into packets that are forwarded independently and share network links with other traffic.Foundational
• Partial MeshA topology in which some nodes have multiple connections but not every node connects to every other.Intermediate
• Passive InterfaceAn interface on which a routing protocol advertises the connected network but does not send hellos or form adjacencies.Foundational
• Passive Optical Network (PON)A point-to-multipoint fiber network that uses unpowered optical splitters to serve multiple customers from one provider port.Intermediate
• Patch CableA short, flexible, factory-terminated cable that connects devices to jacks or patch panels to switches.Foundational
• Patch PanelA panel of ports in a rack where permanent cabling terminates, allowing short patch cables to connect runs to switches.Foundational
• Path MTU Discovery (PMTUD)A technique a host uses to find the smallest MTU along a path by sending packets with fragmentation disallowed and reacting to ICMP errors.Intermediate
• Path-Vector Routing ProtocolA routing protocol class that advertises the full sequence of autonomous systems a route has traversed, used for loop prevention.Intermediate
• PathpingA Windows command that combines traceroute and ping to show loss and latency at each hop over a period of time.Intermediate
• Peer-to-Peer Network (P2P)A network in which devices share resources directly with each other without a dedicated central server.Foundational
• Per-Hop Behavior (PHB)The forwarding treatment a DiffServ router applies to packets based on their DSCP marking.Intermediate
• Per-VLAN Spanning Tree Plus (PVST+)Cisco's STP variant that runs a separate 802.1D instance per VLAN, allowing per-VLAN root placement and load sharing.Foundational
• Physical Layer (L1)OSI Layer 1, which transmits raw bits as electrical, optical or radio signals over a medium.Foundational
• Physical TopologyThe actual layout of cables, devices and connections in a network.Foundational
• Plenum-Rated Cable (CMP)Cable with a fire-resistant, low-smoke jacket required for installation in plenum spaces used for air circulation.Foundational
• PoE InjectorA midspan device that adds PoE power to an Ethernet cable when the switch does not provide PoE.Foundational
• PoE Plus (PoE+)IEEE 802.3at, which delivers up to 30 W per port at the switch (25.5 W at the device) over two pairs.Foundational
• PoE Plus Plus (PoE++)IEEE 802.3bt, delivering up to 60 W (Type 3) or 90 W (Type 4) at the source using all four pairs.Intermediate
• Point-to-Point Protocol (PPP)A Layer 2 protocol for direct links that provides framing, authentication (PAP or CHAP) and network-layer negotiation.Intermediate
• Point-to-Point Protocol over Ethernet (PPPoE)A protocol that encapsulates PPP frames in Ethernet, used by many DSL and some fiber ISPs to authenticate subscribers.Intermediate
• Point-to-Point SubnetA subnet sized for exactly two devices, traditionally a /30 and now often a /31 in IPv4 or /127 in IPv6.Intermediate
• Point-to-Point TopologyA direct link between exactly two endpoints.Foundational
• Pointer Record (PTR)A DNS record that maps an IP address to a host name, used for reverse lookups.Foundational
• Poison ReverseA rule where a router advertises a route back toward its source with an infinite metric, overriding split horizon.Intermediate
• PolicingA QoS action that enforces a rate limit by dropping or remarking traffic that exceeds it, without buffering.Foundational
• Port Address Translation (PAT)NAT that maps many inside addresses to one (or a few) public addresses by tracking unique source port numbers; also called NAT overload.Foundational
• Port Aggregation Protocol (PAgP)Cisco-proprietary EtherChannel negotiation protocol with modes desirable and auto.Foundational
• Port ForwardingA NAT rule that maps a public IP and port to an internal host and port so outside clients can reach an internal service.Foundational
• Port Mirroring (SPAN)A switch feature that copies traffic from one or more ports or VLANs to a monitoring port for analysis.Intermediate
• Port NumberA 16-bit number (0-65535) that identifies a specific application or service on a host at Layer 4.Foundational
• Port ScannerA tool that probes a host for open TCP and UDP ports to identify running services.Intermediate
• Port SecuritySwitch feature that limits which or how many MAC addresses can use a port.Foundational
• Port-Channel InterfaceThe logical interface representing an EtherChannel bundle on Cisco devices, where Layer 2 or Layer 3 settings are applied.Foundational
• PortFastA Cisco feature that moves an access port directly to forwarding, skipping listening and learning.Foundational
• Post Office Protocol 3 (POP3)An email retrieval protocol that downloads messages from a server to a client, using TCP 110 (or 995 with TLS).Foundational
• Power over Ethernet (PoE)Delivering DC power and data together over twisted-pair Ethernet cabling, standardized in IEEE 802.3af, 802.3at and 802.3bt.Foundational
• Power Sourcing Equipment (PSE)The device that supplies PoE power, such as a PoE switch (endpoint PSE) or a midspan injector.Intermediate
• Powered Device (PD)A device that receives operating power over Ethernet cabling, such as an IP phone, camera or access point.Intermediate
• Preboot Execution Environment (PXE)A standard that lets a computer boot from the network, using DHCP to find a boot server and TFTP to download a boot image.Intermediate
• Precision Time Protocol (PTP)IEEE 1588, a protocol providing sub-microsecond clock synchronization using hardware timestamping.Advanced
• Prefix Delegation (DHCPv6-PD)A DHCPv6 mechanism through which an ISP assigns a whole prefix, such as a /56, to a customer router for distribution to its LANs.Advanced
• Prefix LengthThe number of leading network bits in an address, written after a slash, as in /24 or /64.Foundational
• Presentation Layer (L6)OSI Layer 6, which translates, formats, compresses and encrypts data so the receiving application can read it.Foundational
• Private IP AddressIPv4 addresses reserved by RFC 1918 for internal use: 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16.Foundational
• Probe RequestA management frame sent by a client to discover nearby APs, either for a specific SSID or any SSID.Foundational
• Process SwitchingThe slowest Cisco forwarding method, where the CPU performs a full routing lookup for every packet.Intermediate
• Protocol AnalyzerA tool that captures and decodes network traffic so it can be examined protocol by protocol.Foundational
• Protocol Data Unit (PDU)The name for the unit of data at a given layer, such as bits, frame, packet, segment or data.Foundational
• Protocol Independent Multicast (PIM)The family of multicast routing protocols that build distribution trees using the existing unicast routing table for RPF checks.Intermediate
• Proxy ARPA router feature that answers ARP requests on behalf of hosts on other networks, letting hosts without a proper gateway still communicate.Advanced
• Proxy ServerA server that acts as an intermediary for requests between clients and other servers.Foundational
• Public IP AddressA globally unique, Internet-routable IP address assigned by an ISP or registry.Foundational
• Punchdown BlockA terminal block where individual wires are terminated by pressing them into insulation-displacement slots.Foundational
• Punchdown ToolA hand tool that seats wires into IDC terminals and trims the excess conductor.Foundational
• QoS Trust BoundaryThe point in the network where QoS markings from endpoints start being trusted rather than rewritten.Intermediate
• Quad Small Form-Factor Pluggable (QSFP)A larger hot-swappable module with four lanes, used for 40G (QSFP+), 100G (QSFP28) and, in QSFP-DD and OSFP forms, 400G and 800G.Intermediate
• Quadrature Amplitude Modulation (QAM)Modulation that varies both amplitude and phase to encode multiple bits per symbol.Intermediate
• Quality of Service (QoS)Tools that classify, mark, queue, shape and drop traffic to give applications appropriate bandwidth, delay, jitter and loss.Foundational
• QUICA UDP-based, encrypted, multiplexed transport protocol (RFC 9000) that underlies HTTP/3.Intermediate
• Radio Frequency (RF)Electromagnetic energy in the roughly 3 kHz to 300 GHz range used to carry data wirelessly.Foundational
• Radio Resource Management (RRM)Automated management of AP channels, transmit power, and coverage based on measured RF conditions.Intermediate
• Rapid PVST+Cisco's per-VLAN implementation of RSTP (802.1w), the default STP mode on current Catalyst switches.Intermediate
• Rapid Spanning Tree Protocol (RSTP)IEEE 802.1w, a faster STP that uses proposal/agreement handshakes, edge ports and alternate ports to converge in seconds or less.Intermediate
• Real-time Transport Protocol (RTP)A protocol that carries real-time audio and video over UDP, with sequence numbers and timestamps for playback timing.Intermediate
• Received Signal Strength Indicator (RSSI)A measurement of received signal power, usually reported in dBm by Wi-Fi radios.Foundational
• Recursive DNS ResolverA DNS server that accepts queries from clients and performs the full lookup process on their behalf, caching results.Intermediate
• Recursive Route LookupResolving a route's next-hop address by performing another routing table lookup to find the outgoing interface.Intermediate
• Reference BandwidthThe bandwidth value OSPF divides by interface bandwidth to calculate cost, 100 Mbps by default.Foundational
• Regional Internet Registry (RIR)One of five organizations that allocate IP addresses and AS numbers within a world region: ARIN, RIPE NCC, APNIC, LACNIC and AFRINIC.Intermediate
• Registered Jack 45 (RJ45)The common name for the eight-position, eight-contact (8P8C) modular connector used on Ethernet twisted-pair cables.Foundational
• Registered PortsPorts 1024-49151, which IANA registers for specific applications and vendors on request.Foundational
• Remote Authentication Dial-In User Service (RADIUS)AAA protocol commonly used for network access, using UDP 1812 (authentication) and 1813 (accounting).Foundational
• Remote Desktop Protocol (RDP)Microsoft's protocol for remote graphical access to Windows systems, using TCP and UDP port 3389.Foundational
• RepeaterA Layer 1 device that regenerates and retransmits a signal to extend the distance a network can cover.Foundational
• Reported Distance (RD)The EIGRP metric a neighbor advertises for its own path to a destination; also called advertised distance.Intermediate
• Request for Comments (RFC)The IETF document series that publishes Internet standards, best practices and informational specifications.Foundational
• RetransmissionResending TCP data that was not acknowledged within a timeout or was reported missing by duplicate ACKs.Intermediate
• Reverse DNS Lookup (rDNS)Resolving an IP address back to a host name by querying PTR records.Intermediate
• Reverse ProxyA proxy that sits in front of servers and accepts client requests on their behalf, often providing TLS termination, caching and load balancing.Intermediate
• Ring TopologyA topology in which each device connects to two neighbors, forming a closed loop through which data travels.Foundational
• RIP Version 2 (RIPv2)Classless RIP that carries subnet masks, supports authentication and multicasts updates to 224.0.0.9.Foundational
• RIPngRIP next generation: the IPv6 version of RIP, using UDP port 521 and multicast FF02::9.Intermediate
• RoamingA client moving its association from one AP to another in the same ESS while keeping connectivity.Foundational
• Rogue Access PointAn unauthorized wireless access point connected to a network, bypassing perimeter security.Foundational
• Rogue DHCP ServerAn unauthorized DHCP server that hands out incorrect addressing, causing outages or enabling on-path attacks.Intermediate
• Rollover CableA cable with pins reversed end to end (1 to 8, 2 to 7 and so on), used to connect a computer serial port to a network device console port.Foundational
• Root BridgeThe switch with the lowest bridge ID, which acts as the reference point of the STP topology; all its ports are designated.Foundational
• Root GuardAn STP feature that prevents a port from becoming a root port; superior BPDUs place it in root-inconsistent state.Intermediate
• Root Name ServerThe servers at the top of the DNS hierarchy that answer queries with referrals to top-level domain servers.Intermediate
• Root Port (RP)The single port on each non-root switch with the lowest cost path to the root bridge.Foundational
• Round-Trip Time (RTT)The time for a packet to travel to a destination and for the response to return.Foundational
• Route PoisoningAdvertising a failed route with an infinite metric so neighbors immediately remove it instead of waiting for it to age out.Intermediate
• Route SummarizationAdvertising a single aggregate prefix that covers multiple more specific routes to reduce table size and contain instability.Intermediate
• Routed PortA physical switch port converted to Layer 3 with no switchport, behaving like a router interface.Intermediate
• RouterA Layer 3 device that forwards packets between different IP networks based on destination IP address and its routing table.Foundational
• Router Advertisement (RA)An ICMPv6 type 134 message routers send periodically and in response to RS, advertising prefixes, the default gateway and configuration flags.Intermediate
• Router ID (RID)A 32-bit identifier, written as an IPv4 address, that uniquely identifies a router within OSPF, EIGRP or BGP.Foundational
• Router on a Stick (ROAS)Inter-VLAN routing using one router interface trunked to a switch with an 802.1Q subinterface per VLAN.Foundational
• Router Solicitation (RS)An ICMPv6 type 133 message a host sends to ff02::2 asking routers to send a Router Advertisement immediately.Intermediate
• Routing Information Base (RIB)The control-plane table holding the best routes chosen from all routing sources, used to build the forwarding table.Intermediate
• Routing Information Protocol (RIP)A distance-vector IGP that uses hop count as its metric, with a 15-hop maximum and 30-second periodic updates.Foundational
• Routing TableThe router's table of best known paths to destination prefixes, built from connected, static and dynamic routing sources.Foundational
• RST FlagThe TCP reset flag that immediately aborts a connection or rejects a connection attempt to a closed port.Intermediate
• Running ConfigurationThe active configuration held in RAM on Cisco IOS devices, distinct from the startup configuration stored in NVRAM.Foundational
• RuntAn Ethernet frame smaller than the 64-byte minimum, usually the result of collisions or faults.Intermediate
• Satellite InternetInternet access delivered via communication satellites, useful in remote areas without terrestrial service.Foundational
• Secure Shell (SSH)An encrypted protocol for remote command-line access, tunneling and file transfer, using TCP port 22.Foundational
• SegmentThe Layer 4 PDU produced by TCP, consisting of a TCP header and a chunk of application data.Foundational
• Sequence NumberA 32-bit TCP header field that numbers the bytes in a stream so the receiver can reorder data and detect loss.Intermediate
• Server Message Block (SMB)The Windows file and printer sharing protocol, running directly over TCP port 445.Foundational
• Service Record (SRV)A DNS record that specifies the host and port for a particular service, with priority and weight.Intermediate
• Service Set Identifier (SSID)The human-readable name of a wireless network, up to 32 octets.Foundational
• Session Initiation Protocol (SIP)A signaling protocol that sets up, modifies and tears down VoIP and video sessions, using port 5060 (unencrypted) and 5061 (TLS).Foundational
• Session Layer (L5)OSI Layer 5, which establishes, manages and terminates sessions (dialogs) between applications.Foundational
• ShapingA QoS action that buffers excess traffic to send it at a configured rate, smoothing bursts.Foundational
• Shared Address SpaceThe 100.64.0.0/10 block reserved by RFC 6598 for carrier-grade NAT between an ISP and customer routers.Intermediate
• Shielded Twisted Pair (STP)Twisted-pair cable with foil or braided shielding around pairs or the whole cable to reduce EMI and crosstalk.Foundational
• Shortest Path First Algorithm (SPF)Dijkstra's algorithm, used by OSPF and IS-IS to compute a loop-free shortest path tree from the LSDB.Intermediate
• Show Interfaces CommandThe Cisco IOS command that displays an interface's status, speed, duplex, counters and errors.Foundational
• Signal-to-Noise Ratio (SNR)The difference in dB between received signal strength and the noise floor.Foundational
• Simple Mail Transfer Protocol (SMTP)The protocol for sending email and relaying it between mail servers, using TCP port 25 (server-to-server) and 587 (client submission).Foundational
• Simple Network Management Protocol (SNMP)A traditional UDP-based protocol for monitoring and managing network devices using MIBs, polling (ports 161) and traps (port 162).Foundational
• Simple Network Management Protocol version 3 (SNMPv3)Version of SNMP adding user-based authentication and encryption for device monitoring and management.Foundational
• Single-Mode Fiber (SMF)Fiber with a very small core (about 9 microns) that carries a single light path, supporting distances of tens to hundreds of kilometers.Foundational
• Sliding WindowA TCP mechanism that lets a sender transmit multiple segments before needing an acknowledgment, with the allowed amount adjusting dynamically.Intermediate
• Small Form-Factor Pluggable (SFP)A hot-swappable transceiver module, typically 1 Gbps, that plugs into a switch or router port to provide fiber or copper connectivity.Foundational
• Small Form-Factor Pluggable Plus (SFP+)An enhanced SFP module supporting 10 Gbps, physically compatible with SFP ports.Foundational
• SNMP TrapAn unsolicited message an SNMP agent sends to a manager on UDP 162 when a significant event occurs.Intermediate
• SNMPv3The secure version of SNMP, providing user-based authentication, message integrity and encryption.Intermediate
• SocketThe combination of an IP address, transport protocol and port number that identifies one endpoint of a connection.Intermediate
• Socket Statistics Command (ss)The modern Linux command that displays socket information, replacing netstat.Intermediate
• Software-Defined Networking (SDN)A network architecture that separates the control plane from the data plane and centralizes control in software controllers with programmable APIs.Foundational
• Solicited-Node Multicast AddressA multicast address (ff02::1:ffXX:XXXX) formed from the last 24 bits of a unicast address, used for efficient address resolution.Advanced
• Spanning Tree Protocol (STP)IEEE 802.1D protocol that prevents Layer 2 loops by electing a root bridge and blocking redundant paths.Foundational
• Spatial Stream (SS)An independent data stream transmitted simultaneously from separate antennas on the same channel.Intermediate
• Spine-and-Leaf ArchitectureA two-tier data center design in which every leaf switch connects to every spine switch, giving predictable, low-latency paths.Intermediate
• Spine-Leaf ArchitectureA two-tier data center network topology in which every leaf switch connects to every spine switch, giving predictable, equal-cost paths.Foundational
• Split HorizonA loop-prevention rule that stops a router from advertising a route out the interface on which it was learned.Intermediate
• Split MAC ArchitectureDivision of 802.11 MAC functions between a lightweight AP (real-time) and a controller (non-real-time).Intermediate
• SSH File Transfer Protocol (SFTP)A secure file transfer protocol that runs as a subsystem of SSH over TCP port 22.Foundational
• Star TopologyA topology in which every node connects to a central device such as a switch.Foundational
• Start of Authority Record (SOA)The DNS record at the top of every zone that names the primary server, administrator email, serial number and timers.Intermediate
• Stateful DHCPv6A DHCPv6 mode in which the server assigns IPv6 addresses and tracks leases, signaled by the RA M flag.Intermediate
• Stateful FirewallA firewall that tracks the state of connections and automatically allows return traffic for sessions initiated from trusted networks.Foundational
• Stateless Address Autoconfiguration (SLAAC)An IPv6 method in which hosts build their own addresses from a router-advertised /64 prefix plus a self-generated interface ID.Foundational
• Stateless DHCPv6A mode in which hosts use SLAAC for addresses but obtain other settings, such as DNS servers and domain name, from a DHCPv6 server.Intermediate
• Static IP AddressAn IP address manually configured on a device that does not change unless an administrator changes it.Foundational
• Static NATA one-to-one, permanent mapping between a private (inside local) address and a public (inside global) address.Foundational
• Static RouteA manually configured route to a destination prefix through a next-hop address or exit interface.Foundational
• Storage Area Network (SAN)A dedicated high-speed network that gives servers block-level access to shared storage.Intermediate
• Storm ControlA switch feature that limits broadcast, multicast or unknown unicast traffic on a port to a threshold.Intermediate
• STP Path CostThe cost STP assigns to a port based on its speed, summed toward the root to choose root and designated ports.Intermediate
• STP Port States802.1D states: blocking, listening, learning, forwarding and disabled; RSTP collapses these to discarding, learning and forwarding.Foundational
• STP TimersThe 802.1D timers: hello time 2 s, forward delay 15 s and max age 20 s, all dictated by the root bridge.Intermediate
• Straight Tip ConnectorA round fiber connector with a 2.5 mm ferrule and a bayonet twist-lock, common in legacy multimode installations.Foundational
• Straight-Through CableA twisted-pair cable with the same pinout (both T568A or both T568B) on each end, used to connect unlike devices.Foundational
• Structured CablingA standardized, hierarchical approach to building cabling that organizes entrance facilities, equipment rooms, backbone and horizontal cabling, and work areas.Foundational
• SubnetA logical subdivision of an IP network defined by a network address and prefix length.Foundational
• Subnet IDThe bits of an IPv6 address between the global routing prefix and the 64-bit interface ID, used to number subnets within a site.Intermediate
• Subnet MaskA 32-bit value with contiguous 1 bits marking the network portion of an IPv4 address and 0 bits marking the host portion.Foundational
• SubnettingDividing a larger IP network into smaller subnets by borrowing host bits for the network portion.Foundational
• Subscriber ConnectorA square fiber connector with a 2.5 mm ferrule and push-pull latching, common in older enterprise and FTTH installations.Foundational
• SuccessorThe EIGRP neighbor offering the lowest-metric loop-free path to a destination; its route is installed in the routing table.Intermediate
• SupernettingCombining multiple contiguous networks into one larger prefix with a shorter mask.Intermediate
• SwitchA Layer 2 device that forwards frames to specific ports based on destination MAC addresses it has learned.Foundational
• Switch StackingConnecting multiple switches with dedicated stack cables so they operate as one logical switch with one management plane.Intermediate
• Switched Virtual Interface (SVI)A virtual Layer 3 interface on a switch representing a VLAN, used for inter-VLAN routing or management.Foundational
• SyslogA standard for sending event log messages to a central collector, traditionally over UDP port 514.Foundational
• Syslog Severity LevelsThe eight syslog message levels from 0 (Emergency) to 7 (Debugging).Intermediate
• T1 LineA North American digital circuit carrying 1.544 Mbps as 24 time-division channels of 64 Kbps.Foundational
• T568AA TIA wiring standard for RJ45 terminations with pin order green-white, green, orange-white, blue, blue-white, orange, brown-white, brown.Foundational
• T568BA TIA wiring standard for RJ45 terminations with pin order orange-white, orange, green-white, blue, blue-white, green, brown-white, brown.Foundational
• Tail DropThe default congestion behavior where packets arriving at a full queue are discarded.Foundational
• Target Wake Time (TWT)An 802.11ax power-saving feature in which client and AP negotiate scheduled wake times.Intermediate
• TCP Connection TerminationThe FIN and ACK exchange (usually four segments) that gracefully closes a TCP connection in each direction.Intermediate
• TCP FlagsSingle-bit control fields in the TCP header (SYN, ACK, FIN, RST, PSH, URG, ECE, CWR) that signal connection state and handling.Intermediate
• TCP Global SynchronizationA condition where many TCP flows experience drops simultaneously, all reduce their windows, then ramp up together, causing oscillating link utilization.Intermediate
• TCP/IP ModelThe four-layer model of the Internet protocol suite: Link (Network Access), Internet, Transport and Application.Foundational
• tcpdumpA command-line packet capture tool for Unix-like systems.Intermediate
• TelnetA legacy protocol for remote command-line access over TCP port 23 that sends everything, including passwords, in clear text.Foundational
• Terminal Access Controller Access-Control System Plus (TACACS+)Cisco-developed AAA protocol over TCP 49 that encrypts the full payload and separates authentication, authorization, and accounting.Foundational
• Text Record (TXT)A DNS record that holds arbitrary text, widely used for domain verification and email authentication (SPF, DKIM, DMARC).Foundational
• Three-Tier ArchitectureA hierarchical campus design with core, distribution and access layers, each with distinct roles.Intermediate
• Three-Way HandshakeThe SYN, SYN-ACK, ACK exchange TCP uses to establish a connection and synchronize sequence numbers.Foundational
• ThroughputThe actual rate of successful data delivery over a link or path, usually lower than the rated bandwidth.Foundational
• TIA/EIA-568 StandardThe TIA standard family for commercial building telecommunications cabling, defining cable categories, distances, pinouts and installation practices.Foundational
• Time ExceededICMP type 11 message sent when a packet's TTL reaches zero (code 0) or fragment reassembly times out (code 1).Intermediate
• Time to Live (TTL)An 8-bit IPv4 header field decremented by each router; when it reaches zero the packet is discarded to prevent endless looping.Foundational
• Time-Domain Reflectometer (TDR)An instrument that sends a pulse down a copper cable and times reflections to locate breaks, shorts, kinks and impedance changes.Intermediate
• Tone Generator and ProbeA two-part tool for tracing cables: the generator puts a tone on a wire and the inductive probe detects it at the other end.Foundational
• Top-Level Domain (TLD)The last label in a domain name, such as .com, .org or a country code such as .uk.Foundational
• Top-of-Rack Switch (ToR)A switch installed at the top of each server rack, connecting the rack's servers with short cables and uplinking to aggregation or spine switches.Intermediate
• TracerouteA tool that discovers the routers along the path to a destination by sending probes with increasing TTL values.Foundational
• TransceiverA device that both transmits and receives signals, typically a pluggable module converting electrical signals to optical or copper media.Foundational
• Transmission Control Protocol (TCP)A connection-oriented Layer 4 protocol that provides reliable, ordered, error-checked delivery of a byte stream.Foundational
• Transport Layer (L4)OSI Layer 4, which provides host-to-host delivery, segmentation, port addressing and optionally reliability.Foundational
• Transport Layer Security (TLS)The cryptographic protocol that secures application traffic with encryption, integrity and certificate-based authentication; successor to SSL.Foundational
• Trivial File Transfer Protocol (TFTP)A simple, unauthenticated file transfer protocol using UDP port 69.Foundational
• Trunk PortA switch port that carries multiple VLANs over one link, identifying them with 802.1Q tags.Foundational
• TunnelingEncapsulating one protocol's packets inside another protocol so they can cross a network that does not support them natively.Intermediate
• Twisted PairCopper cabling in which pairs of insulated wires are twisted together to cancel electromagnetic interference and crosstalk.Foundational
• Unequal-Cost Load BalancingDistributing traffic across paths with different metrics, natively supported by EIGRP through the variance command.Intermediate
• UnicastOne-to-one communication from a single sender to a single receiver.Foundational
• Unique Local Address (ULA)IPv6 addresses in fc00::/7 (in practice fd00::/8) meant for local use within a site or group of sites, similar to IPv4 private addresses.Intermediate
• Unknown Unicast FloodingForwarding a frame out all ports in its VLAN (except the source) when the destination MAC is not in the MAC table.Intermediate
• Unlicensed National Information Infrastructure (U-NII)FCC-defined unlicensed sub-bands in 5 GHz (U-NII-1 to U-NII-4) and 6 GHz (U-NII-5 to U-NII-8).Intermediate
• Unshielded Twisted Pair (UTP)Twisted-pair cable without metallic shielding, relying only on twisting to reject interference; the most common LAN cable.Foundational
• Unspecified AddressThe IPv4 address 0.0.0.0, meaning "this host" or "no address," used as a source before an address is assigned.Intermediate
• Usable Hosts FormulaThe calculation 2^h - 2, where h is the number of host bits, giving the assignable IPv4 addresses in a subnet.Foundational
• User Datagram Protocol (UDP)A connectionless Layer 4 protocol that sends datagrams with minimal overhead and no delivery, ordering or retransmission guarantees.Foundational
• User Priority (UP)The 802.1D-based 0-7 priority value carried in 802.11 QoS data frames.Advanced
• Variable Length Subnet Mask (VLSM)Using different subnet mask lengths within the same address space to match each subnet's size to its host requirement.Intermediate
• Virtual Local Area Network (VLAN)A logical Layer 2 broadcast domain created on switches, independent of physical location.Foundational
• Virtual Private Network (VPN)Encrypted tunnel that securely connects remote users or sites across an untrusted network.Foundational
• Virtual Router Redundancy Protocol (VRRP)IETF-standard FHRP (RFC 5798 for VRRPv3) with one master and backups sharing a virtual IP that may be a real interface address.Foundational
• VLAN Trunking Protocol (VTP)A Cisco protocol that propagates VLAN definitions from a server to other switches in the same VTP domain.Intermediate
• VLAN Trunking Protocol PruningA VTP feature that stops flooding of broadcast and unknown traffic for a VLAN across trunks to switches with no ports in that VLAN.Intermediate
• Voice over IP (VoIP)Technology that carries voice calls as packets over IP networks instead of traditional circuit-switched lines.Foundational
• Voice QoS RequirementsGuidelines for acceptable voice quality: one-way delay under 150 ms, jitter under 30 ms and packet loss under 1 percent.Foundational
• Voice VLANA separate VLAN on an access port for IP phone traffic, which the phone tags while passing PC traffic untagged.Foundational
• Wavelength Division Multiplexing (WDM)Transmitting multiple signals over one fiber by assigning each a different wavelength of light.Intermediate
• Weighted Random Early Detection (WRED)RED with different drop profiles per IP Precedence or DSCP, so lower-priority packets are dropped earlier.Intermediate
• Well-Known PortsPorts 0-1023, assigned by IANA to common system services such as HTTP, SSH and DNS.Foundational
• Wi-Fi 6EWi-Fi 6 (802.11ax) devices that can operate in the 6 GHz band.Foundational
• Wi-Fi 7Wi-Fi Alliance name for 802.11be devices, emphasizing multi-link, wider channels, and lower latency.Foundational
• Wi-Fi Generation NamesWi-Fi Alliance consumer names mapping standards to generations: Wi-Fi 4 (n), 5 (ac), 6 (ax), 7 (be).Foundational
• Wi-Fi Multimedia (WMM)Wi-Fi Alliance certification of 802.11e EDCA QoS with four access categories.Intermediate
• Wi-Fi Protected Access 2 (WPA2)Wi-Fi Alliance security certification implementing 802.11i with AES-CCMP, in Personal (PSK) and Enterprise (802.1X) modes.Foundational
• Wi-Fi Protected Access 3 (WPA3)Current Wi-Fi security standard that uses SAE for personal networks and mandates protected management frames.Foundational
• Wide Area Network (WAN)A network spanning large geographic areas, connecting LANs across cities, countries or continents, usually over provider services.Foundational
• Wildcard MaskAn inverse mask where 0 bits must match and 1 bits are ignored, used in ACLs and OSPF/EIGRP network statements.Foundational
• Window SizeThe TCP header value advertising how many bytes the receiver is currently willing to accept.Intermediate
• Wireless LAN Controller (WLC)A device or virtual appliance that centrally manages lightweight APs, RF, security, and client policy.Foundational
• Wireless Mesh Network (WMN)A WLAN in which APs use wireless backhaul links to reach a wired-connected root AP.Intermediate
• WiresharkA free, open-source graphical protocol analyzer that captures and decodes network packets.Foundational
• Zone TransferReplication of DNS zone data from a primary to secondary servers, as a full transfer (AXFR) or incremental transfer (IXFR), over TCP 53.Intermediate