"Doom II: Hell on Earth is a first-person shooter computer game originally released in 1994 as the sequel to the immensely popular Doom, which was released a year earlier.
The main additions to the game were the additional monsters available for the player to fight. Doom II doubled the amount of non-boss monster types and started using bosses from the original Doom as normal level enemies. In addition, the multiplayer functionality was greatly improved in Doom II, including "out of the box" support for a vastly increased number of dial-up modems. The two player dial-up connection allowed one player to dial in to the other player's computer in order to play either cooperatively or in deathmatch style combat. There was also LAN functionality added, which was improved upon as patches and updates were released. This functionality was later incorporated into the original Doom."
Source: Wikipedia, "Doom_II," available under the CC-BY-SA License.