In home networking, port forwarding, also called port mapping or punch-through, enables you to create a permanent translation entry that maps a protocol port on your gateway machine to an IP address and protocol port on your private LAN. It’s a transparent process, meaning network clients cannot see that port forwarding is being done. This process enables you to run a public Internet service on a machine that is otherwise hidden from the Internet by your gateway. Port forwarding may also be used to aggregate traffic from an application that uses several ports for transactions and consolidate it into one port for reporting the total traffic identified with that application.
Also see Well-Known TCP Port Numbers in the Quick Reference section of Webopedia.
