Depending the router, adjust the port forwarding or port triggering.
You must set it to allow certain ports, depending on the video game.
You can find this out by doing a simple internet search.
This may not work, but it probably will.
Chances are you need to allow ports for each computer to though.
Sp2 (Windows service pack 2) blocks every single port with their damned firewall, so you'll have to disable that

.
Edit: Also, since this is a lan game. Make sure the right tcp/ip protocols are working on all computers.
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