Port number

A port number is a number designating a channel of communication that is dedicated for use of one particular transmission protocol. For example, if you wanted to connect to this web server to fetch a page, the browser would use the port number corresponding to HTTP transfers, port 80.

Port numbers are used to logically sort the types of connections to a computer, otherwise the computers would have to somehow notify each other of what protocol they wish to use on transmission.

Just as the physical address (MAC address) and the logical address (IP address) helps to route the traffic to a particular machine on a network, it is the port that identifies which service on the machine this traffic is destined to.