What do you do about networks that aren’t subnetted neatly on octet boundaries, like subnetted /24 (Class C-sized) networks? In these cases, you can’t delegate along lines that match the subnets. This forces you into one of two situations: you have multiple subnets per in-addr.arpa zone or you have multiple in-addr.arpa zones per subnet. Neither is particularly pleasing.
Let’s take the case of the /8 (Class A-sized) network 15/8, subnetted with the subnet mask 255.255.248.0 (a 13-bit subnet field and an 11-bit host field, or 8,192 subnets of 2,048 hosts). In this case, the subnet 22.214.171.124, for example, extends from 126.96.36.199 to 188.8.131.52. Therefore, the delegation for that single subdomain in db.15, the zone datafile for 15.in-addr.arpa, might look like this:
200.1 NS ns-1.cns.hp.com. 200.1 NS ns-2.cns.hp.com. 201.1 NS ns-1.cns.hp.com. 201.1 NS ns-2.cns.hp.com. 202.1 NS ns-1.cns.hp.com. 202.1 NS ns-2.cns.hp.com. 203.1 NS ns-1.cns.hp.com. 203.1 NS ns-2.cns.hp.com. 204.1 NS ns-1.cns.hp.com. 204.1 NS ns-2.cns.hp.com. 205.1 NS ns-1.cns.hp.com. 205.1 NS ns-2.cns.hp.com. 206.1 NS ns-1.cns.hp.com. 206.1 NS ns-2.cns.hp.com. 207.1 NS ns-1.cns.hp.com. 207.1 NS ns-2.cns.hp.com.
That’s a lot of delegation for one subnet!
You’d set this up with the DNS console by running the New Delegation Wizard (eight times!) and specifying two labels of the domain name of the delegated domain, as shown in Figure 10-8.