Classless IN-ADDR.ARPA Delegation
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H. Eidnes, G. de Groot, P. Vixie · 1998-03
Abstract
RFC 2317 describes a method for delegating reverse DNS zones for IPv4 prefixes shorter than /24 (e.g., /25, /26, /27) by using CNAME records to redirect PTR lookups into sub-zones managed by the delegatee. This allows ISPs to delegate reverse DNS control to customers who receive less than a full /24 block without splitting the parent /24 zone.
Why This RFC Matters
Before RFC 2317, reverse DNS delegation required full /24 boundaries, meaning ISPs had to maintain PTR records for all customers receiving smaller allocations. This created operational friction and meant customers could not control their own reverse DNS. RFC 2317's CNAME-based delegation pattern solved this elegantly and is now widely deployed by ISPs and hosting providers, enabling self-service PTR record management for end customers.