[UDP] 1200 (Used for Steam’s Friends service)
[UDP] 27000 to 27015 inclusive
[TCP] 27030 to 27039 inclusive
These are all Steam related ports

[UDP] 27015 and 27020 (Default dedicated server port, also used for HLTV)
[TCP] 27015 (For RCON access)
These ports are used by the dedicated server itself

The required ports were found here. You will also need port 27011 outbound allowed (configure your firewall), because this will allow you to connect to the master servers.