So, you want to play deathmatch in MMod? Alright, I'll show you the basic way to get it going.
BUT, first you must be warned about a few things:

• MMod does not support multiplayer officially.
  
• The dev does not plan on supporting multiplayer, so don't complain when things break.
  
• Things WILL break, and you WILL crash!

With the basic warnings out of the way, here are some advance warnings

• Hosting a dedicated server doesn't work.
  
• Beware of the gauss gun, shooting a player with it crashes the game.

Above all, please do not bother the dev about multiplayer being buggy. We will just have the fun trying to do it anyway even though it's not supported.

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With that out of the way, let's begin.

There are a few things we have to prepare, let's start from the easiest.
Since MMod doesn't come with multiplayer maps (for obvious reasons), we will have to bring our own. You could play deathmatch on standard maps, but classic deathmatch maps are way more suited for this.
Thankfully, we can simply copy some classic deathmatch maps from Half-Life 1. Since you need to own Half Life to play MMod, everyone should already have this available (if not, simply install original HL1).
Simply copy desired maps from HL1 maps folder into the MMod maps folder

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This step is kinda optional, but unless you want to use Hamachi or whichever other LAN emulation software is used these days, you'll want to follow do this part.
I won't go into details about port-forwarding here, but you can check this video as a starting point if you're unsure what port-forwarding is and how to do it.


Also check this link for port forwarding instructions
MMod, like other goldsrc games, uses default port 27015, but you can change it to anything using command in the game console.
Keep in mind that port forwarding may not be possible for some people depending on your Internet provider.

Let's do a quick and dirty setup and host this deathmatch game already!
So, launch MMod and press the tilde key (~) to open your console.
After that input these commands.

• maxplayers x - this command changes the maximum number of players in-game that you can have. so change the x to any number greater than 1. maxplayers 1 will turn your game back into singleplayer and disable multiplayer features. Keep in mind that changelevel triggers will stop working in multiplayer, so you won't be able to play through the campaign in coop.
  
• sv_lan 0 - this is what allows other to connect from outside of your local network.

With that done let's start a game. Type in the command map and the name of the map you want to play. In this example, crossfire.

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Once in-game, there are a few things you may want to input into the console.

• mp_weaponstay 1 - This command will make it so once someone picks up a weapon, it will immediately respawn and be available for pickup by other players.
  
• mp_fraglimit 25 - Sets a limit for how many kills are needed for someone to win. After that the game changes to the next map. The maps that will be cycled are defined in \steamapps\common\Half-Life MMod\valve\mapcycle.txt
  
• name StereoBucket - This is something you and all your friends joining should use to change your in-game name. Just use your name, not mine.

Additionally, if you want to change your player model, use model command
Models you can use are found in
With that done, let your friends join. Give them your public ip (google "what's my ip") and they should be able to connect with the connect command in console.
Obviously replace 123s with your own ip.

Enjoy!

Alright, in order for you to not have to type all of those commands in every time, go to folder and edit listenserver.cfg
You will see a few commented out lines (starting with //). For the commands we used before, uncomment them and add the values we used before.

You can tweak settings in here as you'd like, but the two important ones are maxplayers and sv_lan.

When you start MMod, simply type and then type to launch a map (in this case crossfire) and you are ready to let your friends join.

Shooting a player with the the gauss will crash the host and whoever shot it. Fully charged shot may not crash, but anything else will crash! Communicate with your friends to not use it!