From their Website
Set in the zombie apocalypse, Left 4 Dead 2 (L4D2) is the highly anticipated sequel to the award-winning Left 4 Dead, the #1 co-op game of 2008.
L4D 2 servers require 1 port to be open. The port can be safely changed to any other.
| Port | default |
|---|---|
| Game/rcon | 27015 |