What is an (internet) PORT?

A port is one of the consecutively numbered input and output doors on an Internet server where contact can be made with other network devices.

Usually, the server accepts requests from certain services on ports with specific, standardized numbers. This ensures that the transmitting device is immediately understood.

For example, port number 143 is responsible for receiving unencrypted IMAP requests, while port 993 is responsible for receiving encrypted IMAP requests. The same applies to web pages, where port 80 is responsible for unencrypted and port 443 for encrypted transmission.

Theoretically, the server operator can use any port for any service, but it makes sense to stick to the standardization, since otherwise the transmitting device would have to know the correct port in addition to the IP address of the recipient.

See also:
Wikipedia: Port