Whenever i try to port forward it says that i cant use the router ip.?

Im trying to setup a tf2 dedicated server so far no luck the only problem now is that i dont have a proper replay port and when i look online it says its because im not port forwarding properly. I have the right ports I just dont know what to put for the last digits of my ip adress when port forwarding. the port im not allowed using is my default gateway, although I have used it before.