If you are both behind the same router then there should be no problems. If not, then you may potentially have to configure the router to direct traffic to the host port. (port forwarding).

This is standard for most peer to peer games (RTS games use a peer to peer system unlike FPS and MMRPG games that use a client - server architecture) and due to the prevalanecy of firewalls, internet security and routers the best thing we can do is help you configure your system. There are even entire websites and specialized programs created to helping gamers set these sort of things up so its very common. We've included the basics for how to do this in this thread https://forums.sinsofasolarempire.com/?forumid=402&aid=174432 and it also includes links to a great tutorial and a special program that can help set you up with minimal fuss. There are also a lot of players and Ironclad reps on the forums, on ICO and in the sins irc channel (irc.stardock.com /join #sins) who can help you out.