In computer networks, a reverse DNS lookup is the querying technique of the Domain Name System (DNS) to determine the domain name associated with an IP address.
Reverse DNS lookups for IPv4 addresses use the special domain. In this domain, an IPv4 address is represented as a concatenated sequence of four decimal numbers, separated by dots, to which is appended the second level domain suffix.
Internet registries and Internet service providers allocated IP addresses in blocks of 256 or larger octet-based blocks for classes. By definition, each block fell upon an octet boundary. The structure of the reverse DNS domain was based on this definition. However, with the introduction of Classless Inter-Domain Routing, IP addresses were allocated in much smaller blocks, and hence the original design of pointer records was impractical, since autonomy of administration of smaller blocks could not be granted.