You don't have to be connected on the internet to run the game, in fact you can launch it without any emulator.
Find hl2.exe in your games (such as hl2 solo, multiplayer or cs source), copy and past it into your steamapps\common\left 4 dead\, make a shortcut and add
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1. -game left4dead -console
at the end of the target (it's in the shortcut properties)
Or you can use this file, but you won't have the console :
http://cs.rin.ru/forum/viewtopic.php?f=10&t=50752
Launch the game and type connect 192.168.x.x (Your Local Server IP)
You can't use openserverbrowser in the console because it doesn't work (dunno why)
To make a local server you need a dedicated server, download it directly from steam (I think it's free) then launch it in LAN mode and disable the Vac protection.
There it is, now try to connect your client to your server.
Your client will be warned:
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1. The server requires that you be running Steam
And the server in the console log:
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1. Sending hearbeat to master server
Perhaps we can run an local master server to fix it, is it possible?
Ps: With this kind of solution, you can't play in Lan yet but you're able to play solo.
