A term used when some kind of firewall, ip masquerade or proxy is being used to share an IP among multiple computers.

The computers sharing the IP can do a pretty good job of sharing the IP normally, when each is doing its own business with the outside world. The problem comes when the outside world wants to come in-- when you want to run some kind of server, be it a web server, ftp or telnet or ssh for shells or MUDs or whatever. The problem there becomes that there is no way for the outside world to contact one specific computer behind that proxy, because it has no globally-available IP address.

So if this needs to happen, what you do is set up inbound port mapping on the proxy or firewall or proxy or whatever, and it will specify a certain port that is just kind of like a tunnel into the private network sharing the IP-- i.e. you set up port 2000 on the computer that really has the IP to map to port 80 (HTTP) on one of the computers sharing the first computer's IP, and anyone who goes to http://firstcomputer:2000/ will be presented with the page running on the web server of the computer sharing the IP, as the traffic to and from port 80 on the second computer is shuttled back and forth through the first computer's port 2000.