The original Diablo used Blizzard’s Battle.net service for official online multiplayer matchmaking.
Released in 1996, Diablo included both single-player and multiplayer play. Battle.net allowed players to connect over the internet, create or join games, and fight through the cathedral and dungeon together. This helped establish the online community model that became central to later Blizzard games.
Diablo also supported local network play through LAN connections. That means Battle.net was the official internet service, while a group playing on the same local network did not need the same online matchmaking route. The distinction is useful because Diablo’s multiplayer options were broader than one connection method.
Later Blizzard games expanded Battle.net into a larger account and game-services platform. Diablo II and Diablo III continued using Blizzard’s online infrastructure, although their systems and account requirements differed. Battle.net is therefore the specific canonical answer for the original game’s official online multiplayer service.