Networking Fundamentals terms

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• 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
• 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
• 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 LayerThe edge layer of a hierarchical network where end devices connect.Intermediate
• 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
• 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
• Angled Physical Contact (APC)A fiber end-face polish angled at 8 degrees to minimize back reflection, used in PON, FTTH and RF-over-fiber.Intermediate
• 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
• 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
• AttenuationThe loss of signal strength as it travels through a medium, measured in decibels.Foundational
• Authoritative Name ServerA DNS server that holds the official records for a zone and answers queries for it with authority.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
• Backbone CablingCabling that connects equipment rooms, telecommunications rooms and entrance facilities, within or between buildings.Foundational
• BandwidthThe maximum data rate a link or path can carry, expressed in bits per second.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
• BNC ConnectorA bayonet-style coaxial connector with a quarter-turn twist lock, used for video, test equipment, T-carrier and legacy 10BASE2 Ethernet.Foundational
• BridgeA Layer 2 device that connects network segments and forwards frames based on MAC addresses, separating collision domains.Foundational
• 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
• 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 CertifierA test instrument that measures a cabling link against TIA/ISO category standards and produces pass/fail reports.Intermediate
• 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 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
• 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
• 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
• Classful AddressingThe original IPv4 scheme that divided the address space into fixed classes A through E based on the leading bits.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
• 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
• Community StringA shared password-like string used for access control in SNMPv1 and SNMPv2c, sent in clear text.Intermediate
• Congestion ControlTCP algorithms that adjust the sending rate based on signs of network congestion such as packet loss or delay.Intermediate
• 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
• Core LayerThe backbone layer of a hierarchical network that provides fast, redundant transport between distribution blocks.Intermediate
• CRC ErrorAn interface error counted when a received frame's FCS does not match the computed CRC, indicating corruption.Foundational
• CrimperA hand tool that compresses a connector onto a cable so its contacts pierce the conductors, used for RJ45 and RJ11 plugs and coax.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
• 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
• 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
• 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
• 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 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 Code Point (DSCP)A 6-bit value in the IP header used to mark packets for quality-of-service treatment.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
• 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
• 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 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
• East-West TrafficTraffic that moves laterally between servers, workloads or devices within a data center or network.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
• 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
• Ephemeral PortsShort-lived client-side source ports assigned by the operating system for outgoing connections, typically 49152-65535.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
• F-Type ConnectorA threaded screw-on coaxial connector used for cable TV, cable modems and satellite equipment.Foundational
• 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
• File Transfer Protocol Secure (FTPS)FTP secured with TLS, either explicitly via AUTH TLS on port 21 or implicitly on ports 990 (control) and 989 (data).Intermediate
• FirewallA device or software that permits or denies traffic between networks or hosts according to a security policy.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 Check Sequence (FCS)The 4-byte Ethernet trailer containing a CRC-32 value used to detect frame corruption.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
• 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
• 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
• 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 802.3The IEEE working group and family of standards that define wired Ethernet physical layers and MAC.Foundational
• Insertion LossThe total signal loss caused by inserting a component or cable segment into a link, measured in dB.Intermediate
• 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
• 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
• 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 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 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 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
• Keystone JackA modular snap-in jack, typically RJ45 with 110-style IDC terminations, mounted in wall plates or modular patch panels.Foundational
• 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 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 LayerThe lowest layer of the TCP/IP model (also called Network Access or Network Interface) covering OSI Layers 1 and 2.Foundational
• Link-Local AddressAn address valid only on the local network segment and never forwarded by routers.Intermediate
• Load BalancerA device or service that distributes client requests across multiple servers to improve performance and availability.Foundational
• 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
• LocalhostThe hostname that resolves to the local machine's loopback address, 127.0.0.1 or ::1.Foundational
• Logical TopologyThe way data flows through a network and how devices communicate, independent of physical cabling.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 PlugA connector that routes a port's transmit pins back to its receive pins to test the port itself without a network.Foundational
• 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
• 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 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 ConverterA Layer 1 device that converts signals between different media types, such as copper twisted pair to fiber.Foundational
• Mesh TopologyA topology in which devices have multiple interconnections, providing redundant paths.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
• 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
• MulticastOne-to-many communication delivered only to hosts that have joined a particular group.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
• 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
• Near-End Crosstalk (NEXT)Crosstalk measured at the same end of the cable as the transmitter, usually caused by poor terminations.Intermediate
• 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 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
• North-South TrafficTraffic that enters or leaves a data center or network, moving between internal systems and external clients.Intermediate
• NTP StratumThe distance of an NTP server from an authoritative reference clock, from stratum 0 (the clock) to 15; stratum 16 means unsynchronized.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
• 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
• 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 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
• 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
• 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
• 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 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
• 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
• Privacy ExtensionsIPv6 features that generate random, temporary interface IDs so a host's address does not reveal a stable hardware identifier.Advanced
• 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
• 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
• 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
• 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
• QUICA UDP-based, encrypted, multiplexed transport protocol (RFC 9000) that underlies HTTP/3.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
• Recursive DNS ResolverA DNS server that accepts queries from clients and performs the full lookup process on their behalf, caching results.Intermediate
• 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 11 (RJ11)A six-position modular connector, usually wired with two or four contacts, used for analog telephone lines and DSL.Foundational
• 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 Desktop Protocol (RDP)Microsoft's protocol for remote graphical access to Windows systems, using TCP and UDP port 3389.Foundational
• Remote Procedure Call (RPC)A mechanism that lets a program execute code on a remote system as if it were local; Microsoft RPC uses TCP port 135 for its endpoint mapper.Intermediate
• RepeaterA Layer 1 device that regenerates and retransmits a signal to extend the distance a network can cover.Foundational
• 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
• RG-6A 75-ohm coaxial cable with a thicker conductor and better shielding than RG-59, the standard for cable TV and broadband.Foundational
• Ring TopologyA topology in which each device connects to two neighbors, forming a closed loop through which data travels.Foundational
• Riser-Rated Cable (CMR)Cable rated to resist vertical fire spread, used in vertical shafts between floors that are not air-handling spaces.Intermediate
• 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 Name ServerThe servers at the top of the DNS hierarchy that answer queries with referrals to top-level domain servers.Intermediate
• Round-Trip Time (RTT)The time for a packet to travel to a destination and for the response to return.Foundational
• 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 Solicitation (RS)An ICMPv6 type 133 message a host sends to ff02::2 asking routers to send a Router Advertisement immediately.Intermediate
• RST FlagThe TCP reset flag that immediately aborts a connection or rejects a connection attempt to a closed port.Intermediate
• 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
• 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
• 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
• Show Interfaces CommandThe Cisco IOS command that displays an interface's status, speed, duplex, counters and errors.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
• 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
• Smart JackAn intelligent network interface device at the demarc that lets the carrier run remote diagnostics, such as loopback tests, on a circuit.Intermediate
• 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
• 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
• 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
• Split PairA wiring fault in which wires from two different pairs are used together as a pair, so pin-to-pin continuity is correct but twisting is broken.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
• Storage Area Network (SAN)A dedicated high-speed network that gives servers block-level access to shared storage.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
• 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
• 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
• 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/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
• 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
• 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
• Ultra Physical Contact (UPC)A fiber connector end-face polish with a slightly domed flat surface that achieves low insertion loss and good return loss.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
• Universal Plug and Play (UPnP)A set of protocols that let devices discover each other and automatically request port forwards on a home router.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
• 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
• Visual Fault Locator (VFL)A pen-shaped device that shines visible red laser light into a fiber so breaks, tight bends and bad connectors glow visibly.Intermediate
• Voice over IP (VoIP)Technology that carries voice calls as packets over IP networks instead of traditional circuit-switched lines.Foundational
• Wavelength Division Multiplexing (WDM)Transmitting multiple signals over one fiber by assigning each a different wavelength of light.Intermediate
• Well-Known PortsPorts 0-1023, assigned by IANA to common system services such as HTTP, SSH and DNS.Foundational
• Wide Area Network (WAN)A network spanning large geographic areas, connecting LANs across cities, countries or continents, usually over provider services.Foundational
• Window SizeThe TCP header value advertising how many bytes the receiver is currently willing to accept.Intermediate
• Wire MapA test that verifies each conductor is connected to the correct pin at both ends of a twisted-pair cable.Foundational
• 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