NAT Routers Part 1: Client Firewalls
Segment 5: Telling your router about communicating client processes
You can use two distinct methods to tell your router about processes inside your PC that are interested in receiving Internet traffic. The first method works with "client" processes, and the second method is needed for "server" processes.
Actually, the client technique works automatically. Your Internet Browser is a good example of a communicating, client processes that wants to receive IP packets from the Internet. When you activate your browser on your PC, the operating system assigns it a "Communicating Process ID". For our purposes in this discussion, we're going to assume that it is assigned TCP process number 2020
So when that browser process begins operating, it sends a TCP packet, addressed to a web server out on the Internet, saying:
"I am TCP process number 2020 on this PC, and I want to send this packet to my web server's home page, and I expect an answer coming back to me as process 2020."