Reverse DNS

The Reverse DNS system is a subset of the main DNS system that allows Internet users to resolve IP addresses into hostnames. This is the obverse of the most common use (mapping hostnames to IP addresses) of the DNS.

To resolve the hostname for a given IPv4 address one must invert the order of the four octets in the address and append .in-addr.arpa. For example, 192.168.1.19 becomes 19.1.168.192.in-addr.arpa. One then makes a request for a PTR record for this hostname in the DNS.

[edit] Reverse DNS Delegation

Since IP address allocation is handled by Regional Internet Registries, the RIRs are also responsible for managing delegations under the Reverse DNS tree. When an IP address range is allocated, the RIR enters NS records for the appropriate subdomain into its own DNS zone. The operators of the IP address zone may also delegate subdomains of their Reverse DNS zone to their own customers.