PTR is a type of record in DNS system used for reverse DNS lookups (IP address to hostname)
IPv4 reverse resolution
Reverse DNS lookups for IPv4 addresses use a reverse IN-ADDR entry in the special domain in-addr.arpa
. In this domain, an IPv4 address is represented as a concatenated sequence of four decimal numbers, separated by dots, to which is appended the second level domain suffix .in-addr.arpa
. The four decimal numbers are obtained by splitting the 32-bit IPv4 address into four 8-bit portions and converting each 8-bit portion into a decimal number. These decimal numbers are then concatenated in the order: least significant 8-bit portion first (leftmost), most significant 8-bit portion last (rightmost). It is important to note that this is the reverse order to the usual dotted-decimal convention for writing IPv4 addresses in textual form. For example, an address (A) record for mail.example.com
points to the IP address 192.0.2.5
. In pointer records of the reverse database, this IP address is stored as the domain name 5.2.0.192.in-addr.arpa
pointing back to its designated host name mail.example.com
. This allows it to pass the Forward Confirmed reverse DNS process.
IPv6 reverse resolution
Reverse DNS lookups for IPv6 addresses use the special domain ip6.arpa
. An IPv6 address appears as a name in this domain as a sequence of nibbles in reverse order, represented as hexadecimal digits as subdomains. For example, the pointer domain name corresponding to the IPv6 address 2001:db8::567:89ab
is b.a.9.8.7.6.5.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.8.b.d.0.1.0.0.2.ip6.arpa
.