- Before everything else:
To deathmatch online, make sure you have the correct versions of Doom, the most important being Doom 2 v1.9. If you don't have version 1.9, visit Doomworld's patches page.
The following programs should be of use to connect to public servers:
Run IDE, set up the proper paths, and you're ready to go. If you don't have the WAD a server is running, when you double-click the game, IDE should automatically fetch the WAD file.
For more information about how to use IDE to join online games, check out this guide.
- What exactly do I need to play Doom or Doom 2 online?
The bare minimum is shareware Doom. It is strongly recommended that you have registered Doom and/or Doom 2 v.1.9 for any sort of community play. ZDaemon (with a large user-base), Skulltag (with many additional features), or Odamex (fully open source) are the engines that can connect you to public servers where other fans may be found playing.
- Where can I play online without a source port?
To get this kind of game going, you're better off asking for opponents or partners on the forums or other such places, as the original game doesn't support connections to public servers through launchers like modern online engines do (see question 4 below).
To connect with your friends, other than using a dial-up modem to participate in a 2-player game through the phone line, you need an application that translates the IPX protocol to TCP/IP. An IPX tunnel allows 2-4 player connections over the Internet with the original executables. If you're using Windows 2000, XP or Vista, DOSBox has an integrated IPX tunnel. Under Windows 9x or ME, you can use an IPX tunnel application called GIT, and another option is Khan. If you use pure DOS, older applications like tcpsetup and ifrag may be of use.
Note also that Chocolate Doom, while a source port, attempts to emulate the original DOS executables as closely as possible, making it another way to play the game in vanilla form, even in multiplayer mode, as it connects directly over TCP/IP (the Internet).
- How do I join a game with csDoom, ZDaemon, Skulltag, or Odamex?
You run the ZDaemon Launcher, Skulltag Launcher, or Odamex Launcher to find servers hosting games. Alternately, you can use IDE: it has the advantage of showing the servers of all the engines on one screen.
You can also play using other source ports, such as ZDoom, PrBoom or JDoom. The issue is, as with the vanilla game, you need to know the other players' IP addresses and the proper command lines to start game. For example, check this entry on the ZDoom Wiki or this thread about connecting PrBoom (GLBoom). In any case, the documentation of each source port usually explains how to get a multiplayer connection running.
- Do I need the full versions of Doom/Doom 2?
You may use the shareware Doom IWAD, but only on servers running games for the shareware version of Doom, or to play with friends that also use shareware. Other than that, nearly all public servers use the retail or commercial games, and community-created add-ons require them, so you are certainly going to have many more levels to play on if you have the full versions.
- Can I play with a 56k modem connection?
Yes, but your ping will be higher unless you're really close to your opponent (geographically), and games may not be as much fun.
- What is the best option if I have a 56k modem?
With various engines (at least ZDoom, PrBoom and Chocolate Doom) you can set the DUP (TICDUP) of the connection, which lowers the necessary bandwidth. It reduces the amount of information sent to each player, so expect game to feel more jerky.
- I need more information. Where can I find some?
More information regarding online Deathmatching can be found at the Doom Wiki and at Doom2.net.
Edit: Updated vanilla section with current information, added info for other "single player" source ports, and made a few other miscellaneous edits in sake or accuracy and currentness. (myk, 02 March 2009)