Reverse DNS resolution (rDNS) is a technique used to determine the hostname associated with an IP address. This is done searching hostname registry and registrar tables for PTR records. RFC 1912 recommends that "for every IP address, there should be a matching PTR record," but it is not a requirement. Consequently, not all IP addresses have a hostname associated with them. In other words, it's a reverse version of the standard "forward" DNS lookup (an IP address from a domain name).