The in-addr.arpa Domain

The in-addr.arpa domain is a conceptual part of the DNS namespace that uses IP addresses for its leaves, rather than domain names. The domain is the part of your zone that enables address-to-name mapping.

in-addr.arpa domain IP addresses are read from lowest level to the root. Thus, the IP addresses are read backward. For example, suppose a host has the IP address 192.168.21.165. In the in-addr.arpa zone files, its address is listed as 165.21.168.192.in-addr.arpa. with the dot at the end indicating the root of the in-addr.arpa domain.