One way to work around this problem is to use an H.323-aware NAT firewall, which can rewrite the
addresses in the signaling payload. Using the proxy in NAT or firewall environments allows administrators to target a single IP address to terminate all H.323 video calls. All incoming and outgoing video calls that access the public network
will use the proxy. With the Cisco Pix Firewall, administrators can enable H.323 fixup and allow UDP
port 1720 traffic to access the IP address of the proxy. If the proxy were not used, the administrator would
have to configure UDP port 1720 to all videoconferencing devices. If you use a Cisco IP/VC 352X or
3540 gateway, port 1820 must be configured for the videoconferencing devices.