Please select the number of players or nodes in the radio list marked N. Then select which player number you are in the list marked P, and finally enter the IP details of the other nodes into the string gadgets.
Most of the current Doom ports have network play ("DeathMatches") implemented. However, the code isn't very efficent yet and is due for a complete rewrite.
I would only recommend playing with maximum 3 people on the Internet at the moment.