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3.51 - Equilibris is not necessarily required. Any version of the game will do, but it must be the same for both players.
Equilibris is simply more balanced; here is a link to it:
http://equilibris.celestialheavens.com/rus/downloads.html
Now, about the game.
Here is one of the common options.
Install Hamachi, get an IP address there, create or find a network (in Hamachi), find your opponent there (online), then write down their IP address, launch the game, go to the network, then join a game, then TCP/IP, enter the opponent's IP address, click refresh, find, and connect.
If you are creating a game, you need to launch Hamachi, go into the game, then go back to the network, then create a game, then TCP/IP, then select a map and wait for the opponent to join (they join using the method described above).