Your router has a bug and only displays the port as forwarded. In this case, you could try to forward the port manually using the router's web interface or upgrade the firmware.
You are trying to connect to the external IP from within your local network. I am not 100% sure, but I don't think that could work. If you want to test the port forwarding, try to connect from another host on the internet.
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I am attempting to use this program to forward 27015 to 192.168.0.29:27015, like so: http://i.imgur.com/fS4ykCS.png
When I nmap this port using 192.168.0.29, I get state of open and I can connect just fine through Garrys Mod.
However, when I attempt to nmap this port using the external IP, the state is closed and I cannot connect through Garrys Mod.
What is the problem here?
Two possible problems come to my mind:
Your router has a bug and only displays the port as forwarded. In this case, you could try to forward the port manually using the router's web interface or upgrade the firmware.
You are trying to connect to the external IP from within your local network. I am not 100% sure, but I don't think that could work. If you want to test the port forwarding, try to connect from another host on the internet.
Ah, number two was correct, it does not work within the local network, but it works fine externally. Thanks!
Last edit: laniner 2013-06-10