The port for IS itself by default is 10101 (TCP), which is shown when you are enabling incoming connections. This is explained in the Using Multiple Computers instructions: "In the General tab of the Configuration window, about halfway down is Uplink Networking. You will need to check the Enable incoming connections on port box, and if you like, you can also change the port number. You may need to open this port (incoming TCP) in your firewall -- there may be a pop-up from the firewall the next time Inner Space is launched."

ISBoxer (for the Multiple Computer Helper) uses port 10102

You may be able to just add exceptions to the Windows Firewall for Inner Space and ISBoxer, otherwise open TCP ports 10101 and 10102 on the "slave" PC(s) through the firewall.