If you look at most older PC games, they have different multi-player modes.
1) Local
2) LAN
3) Internet
Get your game working as a multiplayer project on the *same* machine. You can use this as your 'control' case so to speak. Anything involving networking or the Internet will add layers of complexity to your project, and you'll be left wondering whether or not your game - or the networking code - is what's causing it to break.
If programmed correctly, a single-PC multiplayer game shouldn't be too difficult to network. It wouldn't be a trivial task, obviously - you should still adhere to smart, extensible programming design for any of your projects. But getting it to work without a network connection would be a great step to guaranteeing that it will work when it does.
Case in point: My racing game 'Retro Racer' was offline only, but I designed it in mind so that I could offer downloadable content for it. Once I got offline maps for it working, I extended the code-base to accommodate DLC content.
Even though I had enough work completed in the game, the difficulties of adding simple DLC was enough to cause a slight mental snap. :/
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