DNS on Windows Server 2003, 3rd Edition

Subnet Prioritization

Subnet prioritization is analogous to the BIND resolver’s address-sorting feature. When the resolver receives multiple address records for the same domain name, it examines the IP address in each record and adjusts the order of the records before returning the list to the calling application: any records with IP addresses on the same subnets as the host on which the resolver is running are moved to the top of the list. Since most applications use addresses in the order returned by the resolver, this behavior causes traffic to remain on local networks.

For example, Movie University has two mirrored web servers on two different subnets:

www.movie.edu.   IN  A  192.253.253.101
www.movie.edu.   IN  A  192.249.249.101

Let’s say the resolver on terminator.movie.edu (192.249.249.3) sends a query and receives these records. It sorts the record with address 192.249.249.101 to the top of the list because terminator shares a network with that address.

Note that this behavior defeats the round-robin feature implemented by most name servers. Round robin refers to the name server behavior of rotating the order of multiple address records in successive responses to distribute the load among the servers (again taking advantage of the behavior of most applications to use the first address in the list returned by the resolver). With subnet prioritization enabled, the order of the records is subject to shuffling by the resolver. You can disable subnet prioritization with a Registry ...