DNS on Windows 2000, Second Edition

Subdomains of in-addr.arpa Domains

Forward-mapping domains aren’t the only domains you can divide into subdomains and delegate. If your in-addr.arpa namespace is large enough, you may need to divide it, too. Typically, you divide the domain that corresponds to your network number into subdomains that correspond to your subnets. How that works depends on the type of network you have and on your network’s subnet mask.

Subnetting on an Octet Boundary

Since Movie U. has just three /24 (Class C-sized) networks, one per segment, there’s no particular need to subnet those networks. However, our sister university, Altered State, has a Class B-sized network, 172.20/16. Their network is subnetted between the third and fourth octet of the IP address; that is, their subnet mask is 255.255.255.0. They’ve already created a number of subdomains of their domain: altered.edu, including fx.altered.edu (okay, we copied them); makeup.altered.edu; and foley.altered.edu. Since each of these departments also runs its own subnet (their Special Effects department runs subnet 172.20.2/24, Makeup runs 172.20.15/24, and Foley runs 172.20.25/24), they’d like to divvy up their in-addr.arpa namespace appropriately, too.

Delegating in-addr.arpa subdomains is no different from delegating subdomains of forward-mapping domains. First, they or their departments create three new zones, 2.20.172.in-addr.arpa, 15.20.172.in-addr.arpa, and 25.20.172.in-addr.arpa. The 20.172.in-addr.arpa administrators also need to ...