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Next, you need to ensure your router is properly configured for port forwarding. Port forwarding tells your router where to direct certain requests it receives based on the incoming port. For example, if you have three computers in the network, and have a web server running on port 80 on one of the machines, port forwarding directs incoming visitors to the desired computer.
You can find full step-by-step pictorial walkthroughs for most common routers and applications at PortForward.com.