Reverse DNS lookup
In the context of the
Internet Protocol and the
Domain Name System, a reverse lookup is often referred to simply as
reverse resolving, or more specifically
reverse DNS lookups. Typically, DNS is used to determine what
IP address is associated with a given
hostname; so to reverse resolve a known IP address is to lookup what the associated hostname for it is.
Reverse DNS lookups for
IPv4 addresses use the special domain in-addr.arpa. An IPv4 address is represented in the in-addr.arpa domain by a sequence of bytes in reverse order, represented as decimal numbers, separated by dots with the suffix .in-addr.arpa. For example, the reverse lookup domain name corresponding to the IPv4 address 101.102.103.104 is 104.103.102.101.in-addr.arpa. A host name for 1.2.3.4 can be obtained by issuing a DNS query for the
PTR record for that special address 4.3.2.1.in-addr.arpa.
Reverse DNS lookups for
IPv6 addresses use similarly the special domain ip6.arpa. An IPv6 address is represented as a name in the ip6.arpa domain by a sequence of
nibbles in reverse order, represented as hexadecimal digits, separated by dots with the suffix .ip6.arpa. For example, the reverse lookup domain name corresponding to the IPv6 address 4321:0:1:2:3:4:567:89ab is b.a.9.8.7.6.5.0.4.0.0.0.3.0.0.0.2.0.0.0.1.0.0.0.0.0.0.0.1.2.3.4.ip6.arpa.
Reverse DNS is important in the role of establishing a given IP address with a specific domain, especially in the cable industry where all IPs resolve to the default cable provider domain. Reverse DNS is used to authenticate an address with the domain.
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How Reverse DNS Works*
Reverse DNS Lookup Tool
