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Networking Fundamentals · Foundational

Port Number

A 16-bit number (0-65535) that identifies a specific application or service on a host at Layer 4.

Definition

TCP and UDP use source and destination ports to deliver data to the right process. Servers listen on fixed, usually well-known ports, while clients use random ephemeral source ports. IANA maintains the official port registry.

Example

An HTTPS server listens on TCP 443 while the client uses a source port such as 52344.

Exam tip

Port questions are guaranteed on Network+, A+ and CCST; memorize common ports with their transport (TCP, UDP or both).

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