If you're wanting to play Left 4 Dead games with freinds, and prefer to use your own server in doing so, this video will show you how to easily set up a Left 4 Dead server using Hamachi. Hamachi is a program that essentially acts as a LAN, only you don't need to be sharing the same internet connection.
Once your friends join your Hamachi network, they simply need to connect to your Hamachi IP. If your Left 4 Dead server is all set up and ready to go, they shouldn't have any problems getting into your game.
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