For a computer connected to a network with an IP address, a port is a communication endpoint. Ports are designated by numbers, and below 1024 each port is associated by default with a specific protocol.
For example, the default port for the HTTP protocol is 80 and the default port for the HTTPS protocol is 443, so a HTTP server waits for requests on those ports. Each Internet protocol is associated with a default port: SMTP (25), POP (110), IMAP (143), IRC (194), and so on.
- Port on Wikipedia