Just passing this info along in case there are others out there running L4D2 Dedicated Server on Windows. Whenever I attempted to start the server I was getting this message in the server console...
Code:
Unable to load Steam support library.
This server will operate in LAN mode only.
...and if a client attempted to connect they would get the error...
Code:
Error: Local games are limited to Class C networks
I was able to resolve this issue by installing the Steam CLIENT on the same drive as my L4D2 Dedicated Server, but different directory. After installing, I ran the CLIENT and it needed to update. Since this was the first time I ever installed Steam CLIENT on this system, I also had to setup Steam Guard.
Now I can start my L4D2 Dedicated Server without issue and players can connect via lobby or direct as normal. Hope that helps.