How would I setup a 2 man coop game in PC Doom?

This probably sounds stupid, but I've only ever done coop in the PSX Doom, I have absolutely no idea how to set it up in the PC Doom. Do the 2 computers have to be connected with a cable (like the PSX Doom), a LAN, or the internet? Do things have to be added to the command line parameter? My preferred port for these purposes is GLBoom, if that's relevant.

The computers can be networked (e.g., a LAN) or can be linked through the Internet. PrBoom uses a server (prboom-server.exe), and the documentation lists its command line parameters (use a batch or shortcut, the parameters are similar to the standard ones but shorter, usually the dash and one character). The server is launched on one computer, and then each client connects (possibly including a client on the same computer as the server). The client user connects by typing simply this:

glboom -net [ip]

Where the IP is the IP of the server computer (can be an Internet IP, local network IP, or a local computer name). Note that only the server loads the parameters, including what wad to use. That means that each client has to have any PWADs in an equivalent directory (probably the one where the EXE is).

Will this work if both computers are running on the same DSL?

When I tried earlier, each client would get this message, but nothing further would happen.

Are they networked (connected to each other)? If so the one running the server would connect to localhost (or 127.0.0.1) and the other to the (local) IP of the server computer (or a name that represents it, if any). If you're trying to connect the clients to your Internet IP then it won't work. You won't be using the DSL IP unless a friend connects to a server you are running on your computer from his house (or whatever).

Ok, using "-net localhost" I was able to get my copy of PRBoom to connect to the PRBoom-server that I'm running on my computer, but I still can't figure out how to get my brother's computer to connect to it. How do I find out what the (local) IP of my computer is?

Both computers are running simultaneously on the same DSL (same IP address), only a few feet away from each other.

The DSL connection isn't relevant; the computers need to be interconnected by a hub or other method of local interconnectivity.

Check here (where it says "Troubleshooting basic connectivity" it explains how to find the computer's IP, on the 3rd step).

Thanks, I got it working now.