Back when Doom was the hottest game on the market, everyone referred to multiplayer games as "deathmatches". When Duke Nukem 3d hit, instead of calling them "deathmatches", they called them "dukematches" for obvious reasons. So when you play someone else either by modem, network, or serial connection, you are playing a "dukematch". Simple enough. 

Now setting up a "dukematch" is alittle more complicated. There are 3 ways to play. 

1)Modem-to-Modem:Ok, I'll try to make this easy enough for anyone to understand. Modem to Modem play is alittle complex. For starters, find a friend that wants to play with you. As long as you both have modems that are at least 14.4, you can play. One person must call, and one person must wait. To start, execute setup.exe in the duke directory. Choose modem game. If it your first time play a modem to modem game, you will need to setup your "COM port" and "modem". I cannot help you with this. You hopefully know what type of modem you have and where it is on your computer(port). My modem is a US Robotics on COM port 2. If all else fails, try the "generic" modem settings with all com ports. When you are done setting up your COM port and Modem, click either "dial opponent" or "wait for call". If you are waiting, just make sure the dot is next to "wait for call" and hit "launch duke nukem 3d". It should connect when your friend calls and start. If you are calling, make sure the dot is next to "dial your opponent", edit the "phone number list", and hit "launch duke nukem 3d". It should call your opponent, connect, and start. If you still can't get your game connection to work, E-MAIL me. HAPPY DUKEMATCHING!
 

2a)Network connection: A local network game is simple to setup. If you are advanced enough to make a local network, I doubt you should need this help. Never-the-less, here we go. Make sure you have at least 2 computers setup in your network(computers hooked together). Execute setup.exe in your Duke3d directory and select "network game". You and your opponents must choose a network socket number. Anything is usually fine. I just use 0x1234 since it is simple. Any combination of numbers works (up to 4 numbers). When you know you all have the same socket number, make sure the number of players set is the actual number of people playing. I know you can count folks. 2 players=2!!! Then just hit "launch duke nukem 3d". If you still can't get your game connection to work, E-MAIL me. HAPPY DUKEMATCHING!
 

2b)Fake a network with Kali!: If you are like me, you don't have a whole bunch of computers hooked together. So whats to do? Well, a man by the name of Jay Cotton came up with a solution. If you could trick your computer into thinking it was on a huge network through the internet, you could play billions of people! Well, there is program that does just this. Kali. It is a program that runs in either Dos, Windows 95, OS2-Warp, or now on a Mac. I use this program constantly to play Duke Nukem 3d. You can download the demo and get information on how to set it up at http://www.kali.net/. I suggest the Windows 95 version if you have Windows 95. The demo has a time limit of 15 minutes (NO!!), but for a one time payment of $20 little bucks, you get a serial number and code to get unlimited use of this program! $20 for the rest of your now meaningless life! Not only does KALIi work with Duke, but also any network capable game! That is a lot of games!! Go get Kali now!!! I go by CpuJunky on Kali so if you see me, give me a yell. If you think your really good join Case's Ladder at http://www.igl.net/duke. Its a great way to see how you compare to other dukers. If you still can't get your game connection to work, E-MAIL me. HAPPY DUKEMATCHING!
 

3)Serial connection: Well, I have had no experience setting up a serial game. Just execute setup.exe from your duke3d directory and select serial game. If this is your first time playing a serial game you will need to setup your COM port. After that just hit "launch duke nukem 3d". I guess it goes then. I don't know. Don't look at me. I don't have a serial connection alright! Give me a break!