What Port Numbers Are For

A single device often runs several network services at once — a web server, a mail server, and a file share. Port numbers let one IP address host many services by giving each a separate doorway.

The IP address gets the packet to the right host; the port number gets it to the right application on that host.

Combine an IP address with a port number and you get a socket, the unique endpoint of a connection. For example, 192.168.1.10:443 is a socket for HTTPS on that host.

A connection is identified by the pair of sockets — source and destination — which is how a host keeps thousands of simultaneous conversations separate.