A reverse DNS lookup is a DNS query for the domain name associated with a given IP address. This accomplishes the opposite of the more commonly used direct DNS lookup, in which the DNS system is asked to return an IP address.
The Internet Engineering Task Force (IETF) standards suggest that all domains should be able to perform reverse DNS lookups, but since reverse lookups are not critical to the normal operation of the Internet, they are not a strict requirement. As such, reverse DNS lookups are not universally adopted.
How does reverse DNS work?
Reverse DNS lookups query DNS servers for a PTR (pointer) record; If the server does not have a PTR record, it cannot resolve a reverse lookup. PTR registers store IP addresses with their segments reversed and add ".in-addr.arpa" to it. For example, if a domain has an IP address of 192.0.2.1, the PTR record will store the domain information at 1.2.0.192.in-addr.arpa.
In IPv6, the latest version of the Internet Protocol, PTR records are stored within the domain '.ip6.arpa' instead of '.in-addr.arpa'.