Domain Name System (DNS) is a mechanism which allows searching for a domain name by its IP address and vice versa, as well as other information that resource records contain. For more information please refer to the article Resource record.
A PTR record of the reverse domain zone is used to define a domain name by IP address. Some Internet services require that IP addresses must be converted into names. To execute the request, the node address is converted to the reverse form. The translation method depends on the IP version.
An IPv4 address consists of four octets of the address expressed individually in decimal numbers from 0 to255, separated by periods, e.g. 192.168.0.1. "in-addr.arpa." domain is used to search for domain names by Ipv4-addresses. An IP-address is specified in the domain name in the reverse order. E.g. 220.127.116.11 corresponds to the domain name "18.104.22.168.in-addr.arpa.".IPv6 address is represented as eight groups of four hexadecimal digits, each group representing 16 bits (two octets) from 0 to ffff, separated by colons, e.g., 3107:0c38:0a67:0000:0000:e446:3925:0091. "ip6.arpa." domain is used to search domain names by Ipv6-addresses. An IP-address is specified in the domain name in the reverse order. E.g., 3107:0c38:0a67:0000:0000:e446:3925:0091 corresponds to the domain name "22.214.171.124.126.96.36.199.6.4.4.e.0.0.0.0.0.0.0.0.7.6.a.0.8.3.c.0.7.0.1.3.ip6.arpa.".