Important note for the host

Starting offline (non-steam) games

If you are not logged into steam when you start a game, it will be advertised on your local network.

Players can join your game via the Join Game menu, or alternatively, by loading any map, and typing into chat: “`open <your IP>” and pressing enter.

You can play non-steam games over the internet if you have port forwarding setup correctly. I believe this port is 7777 (game) and 27015 (server query), UDP.