Traditionally, subnet delegations were intended to fall along subnet classes defined by the number of octets shared in common. Under this system the smallest subnet that could be created was the class C subnet with 256 IP addresses of which 254 are usable. However as time has passed and the demand on IP addresses has grown dramatically, it is no longer practical for providers to devote 256 addresses to customers only intending to use six or seven.
Classless subnets are delegated in the reverse zone of the parent that is delegating. The key tools for the delegation are PTR records, NS records and CNAME records. The NS records declares the existence of the subnet’s domain name servers. The PTR records attach canonical names to reverse lookup addresses. CNAME records can be used to create aliases for simplification.
The ways that these record types can be used to generate classless subnets varies from the crude:
To the elegant:
18.104.22.168.in-addr.arpa. CNAME 9.example.com.
- RFC2317 Classless IN-ADDR.ARPA delegation
- RIPE Reverse Delegation How To
- Classless in-addr.arpa. Delegation