A special domain in the Domain Name System (DNS) that is used for inverse queries. Using the in-addr.arpa domain, a resolver can submit a request to a name server to resolve an IP address into its corresponding fully qualified domain name (FQDN).
The in-addr.arpa domain contains nodes whose names are based on IP addresses with octets in the reverse order. For example, a host with the IP address 172.16.8.44 would be represented in the in-addr.arpa domain by 18.104.22.168.in-addr.arpa.
Resource records for the in-addr.arpa domain are called pointer (PTR) records and are contained within a type of zone file called a reverse lookup file.