1. <networking> A logical channel or channel endpoint in a communications system. The Transmission Control Protocol and User Datagram Protocol transport layer protocols used on Ethernet use port numbers to distinguish between (demultiplex) different logical channels on the same network interface on a computer.
Each application program has a unique port number associated with it, defined in /etc/services or the Network Information Service "services" database. Some protocols, e.g. telnet and HTTP (which is actually a special form of telnet) have default ports specified as above but can use other ports as well.
Some port numbers are defined in RFC 3232 (which replaces RFC 1700). Ports are now divided into: "Well Known" or "Privileged", and "Ephemeral" or "Unprivileged" (comprising "Registered", "Dynamic", "Private").