Pottercraft

Hello everyone! I have tekkit for like 2 months now and i wanted to play with my sister on my own singleplayer world. This is what I did and what happened: - I did ESC - I clicked on Open to LAN - Then I saw Port: and IP: - I didn't know what that was so i clicked on start LAN-World. - My sister couldn't join and then i saw in the chat: Error: Make sure you have a correct IP entered. What is this and how do I get a good IP?