Hey guys,
I was having the same problem as many of you and I was unable to play with my buddy over Hamachi because we kept getting messages about how LAN games are restricted to the same local network.
The person running the server would see, "Connection Refused #Valve_Reject_LAN_Game" in the console.
I HAVE FOUND A SOLUTION. As someone mentioned earlier you need to be in the same Subdomain in order for L4D to believe that you are all in the same network.
This means that if your Hamachi IP is 5.10.20.30 then all your friends need to have Hamachi IPs that begin with 5.10.xx.xx.
Hamachi IPs are given out as people sign up for the service. Me and my 2 other friends were able to get very similar Hamachi IPs by deleting our Hamachi preferences and then relaunching Hamachi. This causes you to have to re-register and it's very likely you'll all get IPs that are in the same range.
Preferences are located in C:\documents and settings\[LOCAL USER]\Application Data\Hamachi if you are running xp,
and \Users\[LOCAL USER\AppData\roaming\hamachi if you're on vista....
Just drag the whole Hamachi folder to your Recycle Bin and launch the Hamachi again.
That's it! You should now have very similar Hamachi IPs and L4D should believe you are on a LAN.
To clarify some of the instructions further up. You DO NOT need to have steam installed to play this game over LAN with Hamachi.
I use the following commands to start my server:
sv_lan 1
z_difficulty hard/normal/easy
net_start
map l4d_airport/farm/hospital/smalltown
sv_allow_lobby_connect_only 0
Then each client simply types:
connect {server hamachi IP]
Good luck and have fun playing! I hope this helps some of you cause I spent like 5 hours yesterday trying to fix this problem.