Sharing an Internet connection

Software — special application configuration

On a network that uses sharing software, all of the client computers have the same public identity because they all share the same public IP address. For most Internet activities, this isn’t a problem because the sharing software always keeps track of which client computer begins a certain connection. However, when an outside computer tries to start a connection to one of the client computers, confusion ensues. Since they all share the same public identity, the sharing software doesn’t know which computer to target, and a connection never forms. In Internet-sharing lingo, “special applications” require this sort of outside-born connection.

Sharing software accomodates these applications with port forwarding. To understand the process, you have to understand that all of a computer’s incoming and outgoing transmissions occur through logical channels called ports. Certain applications and protocols use certain ports. Web servers, for example, use the HTTP protocol, which operates on port 80. Thus, when you access "", you are actually accessing port 80 on a computer named "".

Port forwarding works by redirecting unidentified incoming traffic on certain ports to the computer of your choice. This opens a relatively small security hole and allows you to configure multiple programs on multiple computers. For example, if you wanted to run a web server, you would forward port 80 to one of your computers. At the same time, you could forward port 21 to a different computer running an FTP server.

The actual setup is fairly easy. First, check the documentation of the offending program to find out what port(s) it uses. If nothing else, you should be able to find this information on Google. Then, make sure you know the IP address of the target computer. To find out what it is, go to the Run box (Start » Run...). Then type “winipcfg” (Win9x) or type “command” and enter "ipconfig /all" at the prompt (Win2k/XP). You should see a field labeled IP address.

After that, simply go into your sharing software’s configuration (ICS users, download ICSConfig) and look for “Port Forwarding” settings. When you've found the appropriate section, type in the port (or port range) and the IP address of the target computer and click Apply/OK. The changes should take effect when you restart your sharing software.