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8 February 2007
Network port numbers provide a way of communicating with the correct application over a network.
For example to view this page your browser knows of this by it's website name, which is then translated into an IP address to find the computer to connect to. See: Basic TCP/IP Network Guide (DNS Section).
Once the browser knows which computer to connect to, there could be several different services running on that computer. For example the server may function as a mail server as well as as serving web page. A port number is used to specify which application, and in this case the web server would use port 80 which is the port used for http.
If you are on a Linux / Unix computer then you can just look in /etc/services to get a list of the well known ports. I have put a list on the web at: Penguin Tutor - TCP and UDP Port Numbers (/etc/services) Quick Reference