A Domain Name System (DNS) query in which a resolver contacts a name server to perform a reverse name lookup, requesting a host name for a given IP address. An inverse query is the opposite of the usual DNS query - that is, given a host’s fully qualified domain name (FQDN), it determines the host’s IP address.
Because of the hierarchical structure of the namespace of the DNS, inverse queries normally have to search all domains to resolve the IP address. To circumvent this, a special domain called in-addr.arpa exists for reverse name lookups. The nodes in this domain are named after the IP addresses of hosts but with the octets in reverse order to facilitate searching. However, inverse queries can take place only on the name server queried and cannot be forwarded to another name server. Because individual name servers manage only a small portion of the entire DNS namespace, there is no guarantee that a given inverse query issued against a specific name server will meet with a successful response.
Most names used on Internet servers are configured for reverse name lookups to avoid the extra overhead required. However, if you need to use the DNS troubleshooting utility nslookup, you should configure the in-addr.arpa domain on name servers to support inverse queries. Otherwise, there is no need to configure this inverse domain.