Reverse DNS lookup is a process to determine the hostname associated with a given IP address. Typically, the 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. A reverse lookup is often referred to simply as reverse resolving, or more specifically reverse DNS lookups.
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... [see Wikipedia]

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