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:
The ISP delegates each IP address as a class D subnet with one or more NS records for each IP address, The customer must create a zone for each IP address, complete with its own SOA record, duplicates the NS records and a PTR record.
To the elegant:
The ISP doesn’t delegate at all, instead using one CNAME record for each reverse IP address in its reverse zone.
126.96.36.199.in-addr.arpa. CNAME 9.example.com.
The 9 attached to Example.com is an arbitrary label, chosen in this case to match the last digit of the reverse IP address. The customer will simply need a PTR record to resolve 9.example.com to a domain name, e.g.
9.example.com. PTR hostname.example.com.