Do you have Linux/FreeBSD hosts on IPv4-only networks? Have you checked your DNS query logs recently? If you have, you’ll might be wondering what all the queries for AAAA records are.
The above table is from a document titled DNS stub resolver behavior of IPv6 ready hosts. It was written in 2006, and the situation remains, at least as far as Linux is concerned.
RHEL/Centos 5.6 and SLES 11 SP1 insist on having its stub resolver query for AAAA records even if IPv6 connectivity is completely disabled in the kernel.
I urge vendors to add a switch to the resolver configuration allowing us to enable or disable this behavior at our discretion.