If you don't want to shell out the money to rent a dedicated server in order to host a Left 4 Dead game server, follow this tutorial on how to create one on your home computer. Keep in mind that if you intend on letting other people play it, you will need a fast connection to prevent lag.
When you create a dedicated Game Server on your machine, lag shouldn't be an issue, so as long as other people aren't playing on it as well.
Valve provides most of the software needed to get started, you will simply need to make some changes and port forwarding to get it public.
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