Are you on the same network as the other computer? You won't be able to access it without the Internet if they are not in the same network.
If they are in the same network,
For Windows:
On the server computer, open up command prompt (Google it if you can't open it).
Type in 'ipconfig' without the quotes.
Look for the part that says 'IPv4 Address . . . . . . . . 192.168.x.x
Type in that num
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