Some of the hurdles that NAT presents to H.323 include:

• H.323 messages, which are part of the IP payload, have embedded IP addresses in them.

  NAT translates the IP address in the IP header, but not the embedded addresses in the H.323 messages. This problem can be and has been addressed with H.323-aware NAT devices. The problem has also been addressed with Communication Manager 1.3 and later versions of the NAT feature.

• When an IP telephone registers with the gatekeeper or call server, the IP address of that endpoint must stay the same for the duration of the registration.

  This hurdle rules out almost all current implementations of many-to-a-pool NAT.

• TCP/UDP ports are involved in all aspects of IP telephony, including endpoint registration, call signaling, and RTP audio transmission.

  These ports must remain unchanged throughout an event, during the registration, or during a call. Also, the gatekeeper must have, ahead of time, the ports that will be used by the endpoints for audio transmission, and these ports can vary for every call. These requirements complicate how H.323 works with port address translation (PAT), which rules out most current implementations of many-to-1 and many-to-a-pool NAT.