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 Domain 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 host address is reversed; 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. 195.161.72.28 corresponds to the domain name "28.72.161.195.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 "1.9.0.0.5.2.9.3.6.4.4.e.0.0.0.0.0.0.0.0.7.6.a.0.8.3.c.0.7.0.1.3.ip6.arpa.".