Yes, If you are on a private network, your firewall or NAT (Network Address Translation) router must be “H.323 aware” or you'll need a SIP proxy if you are using SIP. To help determine if your LAN uses NAT, you can use a web browser and go to the following URL: http://patton.com/support/showmyip
This shows both the public and private IP address of your PC.

Note: H.323 aware routers and firewalls support "snooping", in which the H.323 control channel is continuously examined and session requests are authenticated. Once authenticated, the requested ports to be used for the H.323 session are opened for the duration of the conference. Upon termination of the conference, the ports are immediately closed by the firewall.
This is often referred to as an Application Level Gateway since this operation requires the firewall to be protocol-aware. Your H.323 aware router must support H.323v3. Both the firewall and the NAT/PAT software in your router must be H.323v3 aware.