FOSS gamedev and creative worlds

I'm trying to play this game with a friend on the same network so we both have the same Public IP Address. How do I get this to work? Everytime I start a server, I don't know how to connect it to the other computer.

Use the internal IP: 192.168.x.x, where x.x is a varying number (it depends of the computer; see the administration interface of your router).

How do I find out what that is?

You can use ifconfig if you're on Linux

Thank you so much!