To play Diablo Hellfire: The Hell mod via Internet , you'll have to use Hamachi, a program that makes games believe you are playing on IPX network while really playing over the TCP/IP protocol.
Here's the step-by-step guide on what to do to start playing Diablo: The Hell in Multiplayer mode within 10 minutes:
1. Download Hamachi and Setup Guide from here.
2. Install Hamachi.
3. Open Hamachi. Go to "Configure Hamachi -> Preferences -> System" and uncheck "Automatically update Hamachi".
4. Open "The Hell over Hamachi setup guide.pdf" (second file inside your downloaded archive from step #1) and follow the instructions.
5. In Hamachi push the button "Create or join Networks" and select "Join an existing network". Then in the pop-up window enter the login: "thehellmod" (w/o quotes, exactly as it's written here, those networks are case-sensitive) and the password: "mordor".
6. Now you're connected to Diablo: The Hell network in Hamachi. Look for other online members and tell'em you wanna play.
If you can't join this network then it's likely to be full. In this case I suggest you join 'thehellmod2' network, if it gets full too, then try thehellmod3, thehellmod4, etc. Password for all them is the same (mordor).
Hamachi is no battle.net, here games are pre-arranged. There's a low chance of meeting someone on-line by accident. Arrange games with your buddies and get online with them. Or use this forum to arrange games with the members and play together. That's the way to do it.
If you still have a problem you can't solve, describe it here and remember that the more details you give, the better will be the answer
If you quit playing online or the game at all, please delete yourself from the network so that others won't see all these dead network members
And by the way, if you quit playing this mod, I'd like to know why. Tell me about it or whatever, such info could be useful.