Delegation allows network administrators to provide authoritative reverse DNS on subnets that don’t fall on octet boundaries. For subnets smaller then a class C this is very useful. The main records for the delegation are PTR records, NS records and CNAME records.
A few of things that need to be remembered:
1. These are classless subnets, they don’t align themselves with class A, B, C, D, or E networks very well.
2. There can be only one primary authoritative name server of the class of IP addresses.
3. Since your ISP or upstream provider controls the class of IP addresses their cooperation is needed to provide reverse DNS.
See Also:
Class A
Class B
Class C
PTR Records
NS Records
Cname Records
IP Address