Enter any IP Address and Subnet to do a Reverse DNS Scan/Lookup on. (i.e. 184.108.40.206/30 or 220.127.116.11/255.255.255.240)
Typically, computer networks use the Domain Name System to determine what IP address is associated with a given domain name. So, to reverse-resolve a known IP address is to look up what host and domain name belongs to that IP address.
Network administrators often refer to a reverse lookup as reverse resolving, or more specifically reverse DNS lookup, and is accomplished using a "reverse IN-ADDR entry" in the form of a PTR record.
Reverse DNS lookups for IPv4 addresses use a reverse IN-ADDR entry in the special domain in-addr.arpa. In the in-addr.arpa domain a sequence of bytes in reverse order represent an IPv4 address, encoded as decimal numbers, separated by dots with the suffix .in-addr.arpa. For example, the reverse lookup domain name corresponding to the IPv4 address 10.12.13.140 is 18.104.22.168.in-addr.arpa. A host name for 22.214.171.124 can be obtained by issuing a DNS query for the PTR record for that special address 126.96.36.199.in-addr.arpa.
Historically, internet registries and IP providers allocated IP addresses in blocks of 256. Thus, each block fell upon an octet boundary. This made configuration of the PTR records easy, since the dot separators delimited each block. Today however, IP addresses are allocated in much smaller blocks, and hence the traditional way of configuring a nameserver to perform reverse DNS cannot work. A means of overcoming this problem was devised and published as RFC 2317. It uses a CNAME entry which corresponds to each block.
Reverse DNS lookups for IPv6 addresses use similarly the special domain ip6.arpa. An IPv6 address appears as a name in the ip6.arpa domain as 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.188.8.131.52.184.108.40.206.0.0.3.0.0.0.2.0.0.0.1.0.0.0.0.0.0.0.1.2.3.4.ip6.arpa.
The w3dt Reverse DNS Scan tool is based off sources from Jon Schatz, 2008.