Sharing an Internet connection
Router — special application configuration
Because routers obscure the real IP addresses of individual computers, it is sometimes difficult for servers on the Internet to find computers behind a router. When the connection originates from behind the router (ex: web browsing), the router takes note of the source computer and everything works okay. When the connection comes from outside the router, however, it does not know where to direct the traffic internally and the communication fails. In Internet-sharing lingo, applications that require this kind of communication are called "special applications".
The easiest way to configure these is to use DMZ, a feature on most routers that causes all unidentified incoming traffic to be redirected to a computer of your choice. You’ll find it in the “Advanced” or “Miscellaneous” section of your router’s control panel. Unfortunately, it can’t be used on more than one computer at a time, and it makes the target computer more vulnerable to hacker attacks.
A better solution is port forwarding, which does something similar in a much more segmented way. 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 "cablesense.com", you are actually accessing port 80 on a computer named "cablesense.com".
Port forwarding works by redirecting unidentified incoming traffic on certain ports to the computer of your choice. It opens a smaller security hole, and it allows you to configure 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. Port forwarding doesn’t limit you to a single task.
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. You can look it up in the same way you looked up your NIC MAC address (see configuration page).
After that, simply go into your router’s control panel and look for “Port Forwarding” or “Virtual Server” settings (note that they may be concealed within an “Advanced” subsection). When you've found the appropriate page, type in the port (or port range) and the IP address of the target computer and click Apply/OK. The changes should take effect immediately.