This guide explains how to play MoO2 multiplayer over the internet. Unlike "Network multiplayer guide" it does not require port forwarding and is also more detailed. The instructions are for Steam version, but can be adapted for GOG and other DOSBox based distributions. See non-Steam section for details.
Ensure all parties use the same version. Version number can be seen in main menu screen in bottom right corner. It should be either:
Everyone uses the same version of 1.50 fan patch.
Everyone uses 1.40b23 (Steam and GOG English defaults).
Everyone uses 1.31 and the same language. Playing, e.g. German 1.31 vs French 1.31 is not supported. All non-English versions on Steam and GOG are 1.31.
This step is important, since the game may crash or hang instead of telling you of version conflict.
Find your Master of Orion 2 local files. In Steam you can do it in library by right clicking on Master of Orion 2 -> Properties -> Local Files tab -> Browse Local files. Open dosboxMOO2_single.conf with notepad and first delete all text there, then paste the following instead:
Start the game by server (any player can act as a server). In main menu click MULTI PLAYER, select NETWORK GAME, click START NEW GAME (or LOAD GAME if you are resuming an old game), choose game settings and server name (default MOX-0 might be already taken) and click accept. Tell server name to the client(s) and wait until they join.
Start the game by client(s), in main menu click MULTI PLAYER, select NETWORK GAME, click JOIN NEW GAME and select your server from the list.
When all clients have joined click START NET GAME on server.
For GOG version instructions are the same. Note that since you replace dosboxMOO2_single.conf on step 2, you need to start single player version to play. Multiplayer GOG launcher does not support symbolic server names, so you can't use it for server I mention.
For versions obtained outside of Steam and GOG (e.g. original CD) you'll need to create your own dosbox config and install dosbox to run the game. The simplest setup would be to take config given in step 2 and change mount command to
mount C "C:\path\to\moo2\"
Using orion95.exe (windows 95 build) is possible but not recommended for MP because of heavy lag and the need to use additional software to emulate IPX over internet.