Sorry >.< lol
LAN = Local Area Network
WAN = Wide Area Network (aka internet IP)
If all your computers are on the same network (all plugged into the same router) you must change the IP address on the client to the servers LAN address e.g. 192.168.1.2 then you should be ok. Only people outside of your LAN (not plugged into or wifi'd to your network) have to use the WAN IP Address to connect.
Just so you know how to find your LAN address for your server...
On the server click the start menu, click run, type: cmd, then press enter, when command prompt appires type: ipconfig and press enter it will then show you your LAN address it should look roughly like: 192.168.1.2
Sorry if this is still confusing i'm crud at explaining things sometimes, but if it is helpful...GOOD TIMES!

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