Ports

What is it?

A transport port is a 16-bit number used with an IP address and transport protocol to identify an application endpoint. Ports let many applications communicate concurrently through the same network interface, for example a web server on one port and a DNS service on another.

Why does it matter?

IP directs packets to a host/interface, while ports help the transport layer deliver data to the correct socket or service on that host. A port number alone is not globally unique; the protocol and endpoint address context matter.

Mental model

Think of an IP address as a building and a port as a service desk inside it. TCP and UDP each have their own port space, and client processes often use ephemeral source ports.

How it works

• The 16-bit port field ranges from 0 through 65535.
• Well-known and registered port assignments are conventions managed through IANA; real services may use nondefault ports.
• A connection is identified by a tuple including protocol, local/remote IP addresses, and local/remote ports.
• A listening TCP socket is different from an established TCP connection.
• Port reachability depends on local listeners, routing, firewalls, NAT, and the application's protocol behavior.

Steps

• A browser opens a TCP connection to a web server at destination port 443.
• The client chooses an ephemeral source port.
• The OS demultiplexes incoming segments using the connection/socket tuple.
• The server's listener accepts the connection and the application speaks HTTPS over it.

Real-world usage

• Debugging a service that is not listening on the expected port.
• Understanding ephemeral ports and NAT translation.
• Reading netstat/ss output and firewall rules.

Common mistakes

• Treating a port number as an identity independent of IP and protocol.
• Assuming port 443 guarantees the service is HTTPS.
• Confusing an open TCP listener with UDP reachability.

Interview questions

Prerequisites

• TCP
• UDP

Related concepts

• TCP
• NAT

Continue learning

• TCP Handshake