Port Numbers

Most protocols we use have the concept of port numbers. This allows us to have multiple applications working with the same IP address. For example, I can access the web server at 81.123.123.1 port 80 to get to the web server.

If I access 81.123.123.1 port 443, I can get to the secure web service.

If I access 81.123.123.1 port 22, I may be able to login to SSH.

These are the 16-bit destination ports which present different services from the same IP address.

When I try to access any of these services, I need to provide a 16-bit source port for the information to come back to. There are some loose rules around this, but my machine will normally pick a random port number between 1024-65,536.