I'm not sure about what do you want to do, but it seems that you want to know what is your gateway, and may be open some ports.
In this case, if you are using ubuntu (i think you do), go to system menu->Admin->Network, or something like that, because I have gnome in spanish. You will be prompted for your password.
Select "Ethernet Conection" or something similar, and press properties button. You should find out your gateway.