I just spent 3 hours trying to connect to a friends game over a local network, browsing forums, learning how to open ports, different settings, everything! I finally found a solution that works and thought it would be nice to post here in case there are other people like me trying to figure this out.
The Problem: Host sets up a server and other players in their local network get the dreadful "unable to connect to server" message.
The fix: You need to see if computers in your network can ping each other.
Step by Step: