Open server (OGS)
You can play with the single player charasters, the game is taking place between customers.
Closed server (CGS)
Games will create and play directly on the dedicated gaming server. All the characters in the same stored on the remote server, and as a result, the number of possible cheating decreased significantly.
D2GS is the closed server, which works with PvPGN emulator.
Old version d2gs-110-bin-beta6
Official page of D2GS project - http://d2dev.dlg.cn (not updated since 2004)