Instructions
Launch Diablo II on your host computer. Within the main menu, click the "Other Multiplayer" game option. Select "TCP/IP Connection."
Click "Host Game." The host computer's IP address appears on the screen. Write down the IP address. Enter the game on the host computer, but don't begin playing yet.
Launch Diablo II on the connecting computer. Click the "Other Multiplayer" option. Select "TCP/IP." Click "Join Game." Enter the IP address written down in step 2. Once the connection has been established, you'll see both of your characters on each computer. At this point, you're ready to play the game.