reverselookups

Reverse lookups refer to the process of retrieving information associated with a value by consulting a directory or index that maps data in the opposite direction of a standard lookup. The term is used in several domains, including networking, telephony, and data directories. In networking, the most common example is reverse DNS, which maps an IP address to a canonical host name. This is achieved through PTR records in the DNS, stored under the reserved in-addr.arpa for IPv4 and ip6.arpa for IPv6. The IP address is reversed in the query, and the corresponding PTR record must exist and be controlled by the owner of the host or network.

Reverse lookups are frequently used in email delivery and anti-spam systems, where mail servers perform a reverse lookup on the sending IP to verify identity, and in logs to translate addresses back into human-readable hostnames. In addition to DNS-based reverse lookups, there are reverse IP lookups that attempt to list domain names hosted on a given IP, and reverse phone or address lookups that map phone numbers or addresses to individuals or organizations through directory services or carrier databases. They vary in coverage and accuracy, and some data may be private or outdated.

Limitations include incomplete mappings, dynamic addressing, privacy restrictions, and the possibility of spoofed or misleading results. Forward-confirmed reverse DNS is a best practice in which a hostname’s forward lookup resolves back to the original IP. Related terms include reverse DNS and reverse IP lookup.