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. Для выполнения запроса a node address is converted to a дрес узла переводится в обратный вид, способ перевода зависит от версии IP.
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. 184.108.40.206 corresponds to the domain name “220.127.116.11.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 “18.104.22.168.22.214.171.124.6.4.4.e.0.0.0.0.0.0.0.0.7.6.a.0.8.3.c.0.7.0.1.3.ip6.arpa.”.