said by NormanS:
My guess would be, "Carrier Grade Nat", which is AT&T's plan for implementing IPv6 compatibility. AFAIK, the only corrective action is to drop AT&T for another provider.
It is likely CGN but that has nothing to do with IPv6 compatibility. CGN is due to IPv4 exhaustion and attempting to prolong the life of IPv4.
AT&T offers 6rd so you can have an IPv6 address. Of course servers which you are trying to reach will have to have IPv6 enabled as well.
CGN is why it's so important that every get on the IPv6 bandwagon as soon as possible.