It should be mentioned that using the private IP address method you may have to turn on SMB file sharing on a computer for it to be able to access other computers' games, and for those to access its games, depending on your security settings. Otherwise I ran into an issue where the gameplay did not sync between players after the initial load of the save, only the chat did. (Which is why we didn't catch it initially, only, that too, after beginning recording, which was... :/ ) Turning on SMB fixed it, though.