Make sure the ip address used by the client is not the IP assigned by your router. My IP address for instance is 192.168.1.101, but this address is the same as local host or 127.0.0.1. To find which IP should be used in your game client, http://whatismyip.com/ should do the trick.
And if that's not your problem, then take this advice while port forwarding:
The fourth quadrant of your router-assigned IP address is the one that your router uses to distinguish each computer on the network. At the moment, my PC is 101.
When you're port forwarding, you have to specify which computer on the network needs to have the ports opened. I use Linksys, and mine looks like this:
Hope this helps resolve your problem.