The DNS database also include zone files that use the IP address as a key to find the host name of the machine, enabling IP address to host name resolution. This process is called reverse resolution or more commonly, reverse mapping. Reverse mapping is used primarily to verify the identity of the machine that sent a message or to authorize remote operations on a local host.
The in-addr.arpa domain is a conceptual part of the DNS namespace that uses IP addresses for its leaves, rather than domain names. It is the part of your zone that enables address-to-name mapping.
Just as DNS domain names are read with the lowest level subdomain occupying the furthest left position and the root at the far right, 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 18.104.22.168. In the in-addr.arpa zone files, its address is listed as 22.214.171.124.in-addr.arpa. with the dot at the end indicating the root of the in-addr.arpa domain.