From FreeSpace Wiki
FS2NetD - SCP Multiplayer
FS2NetD is an online FS2 game tracker. It acts as a lobby for multiplayer games, and it tracks registered pilots and mission statistics. When PXO is slated to be back online in 2007, FS2NetD will go away.
Multiplay over TCP/IP
FreeSpace 2 can also be played by connecting directly to a hosting game via their i.p. address.
A dedicated/standalone FreeSpace2 server can be run on the Windows, Linux and Mac platforms. This is enabled with the -standalone option. Currently, standalone servers are beta and undergoing continued testing and development.
On Linux and Mac the standalone server utilises a set of features called WebUI