Usually if your router isn't handicapped, you put in the port aka 27000 then in the 2nd port box you put 27015, then it'll say something like 192.168.1. Here you put your computers IP to your router, ie 100 if its the Main computer on the router network. Then you choose whether its UDP or TCP or Both, if it has like a Name box, thats just to remind you with the port is actually doing so you put something like Gmod.