Hmmmm. Looks like a good FAQ entry here

You don't need to use other programs to run an IP game at all.
How can I play FS2 Online?PXO was the way to play Freespace 1 & 2 online for many years but it has now been taken down meaning that there is no way to play the retail versions of Freespace online unless you know the IP addresses of people playing a game. Fortunately FS2_Open can be played online and this is now becoming the version people use when playing online.
All recent versions of FS2_open support multiplayer but many people prefer to use Tom's Old Build. This is a version of FS2 Open which basically consists of the original FS2 source with the new multiplayer code added. As such it has less of the SCP changes. This has typically resulted in a more stable, bug free build. The drawback is of course that none of FS2_Open's other new features will work meaning that the game will look exactly like the retail version of FS2. A guide on how to get online with Tom's old build can be found on
Game Warden.
Other people like the eye candy that FS2_Open gives them. In addition when multiplayer missions come out using the new features of FS2_Open the old build won't be able to play them. If you want to play using a newer build the
SCP Website has a guide on what to do.
I don't want to use FS2_Open / I want to play on a LANAll versions of freespace support IP play. To play an IP game follow these simple steps.
[LIST=1]
- First thing you'll need is the IP address of all the computers that will be playing.
a) On win9x. Start>Run>Type WinIpcfg into the box and press okay. Note down the IP address.
b) On Win2k\XP Start>Run>Type cmd to open a command prompt. Type IPconfig.
- Once you have your IP address run Freeespace. Press F2 once you've choosen your pilot to get into the options screen then click on multi to configure your mulitplayer options.
- Turn off the light by PXO. Make sure TCP is selected instead of IPX.
- Choose ADD in the IP Address field. Type in the IP Address of the other machine(s). (There is no need to add the address of your own machine).
- Click accept. Go play multiplayer