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Protecting Your Data: A Guide to Windows Firewall
"Well Known" Ports
Servers on the Internet use "well known" ports to communicate. For example, almost every web server in the world uses port 80 as its "listening" port. The web server sits idly until a remote station requests a web page on port 80.
Since client computers are usually initiating a conversation, their origination ports don't have to be "well known" ports. Your client computer chooses a temporary port number for the conversation, usually in numerical order.
In the diagram below, a single computer with the IP address of 192.168.0.6 is using two web browser windows. One web browser is surfing Google, and the other web browser is pointed to Yahoo! Although both browser destination ports are specified as port 80 (the "well known" web services port), the source ports have been assigned port 1625 for the Google browser and port 1626 for the Yahoo! browser. If a page to another site is opened, the source port for this new session would increment again to port 1627. Although the source port numbers are always incrementing, notice that the web service always runs on port 80 and the IP address of the computer never changes.
