Reverse lookup is a network service used to determinate a domain name that is associated with a given IP address using the Domain Name System (DNS) of the Internet. It takes the form of a question, such as “Can you tell me the DNS name of the computer that uses the IP address 192.168.1.1?”
One problem for supporting the reverse query process is the difference in how the DNS namespace organizes and indexes names and how IP addresses are assigned. If the only method to answer the previous question was to search in all domains in the DNS namespace, a reverse query would take too long and require too much processing to be useful.
To solve this problem, a special domain, the in-addr.arpa domain, was defined in the DNS standards and reserved in the Internet DNS namespace to provide a practical and reliable way to perform reverse queries. To create the reverse namespace, subdomains within the in-addr.arpa domain are formed using the reverse ordering of the numbers in the dotted-decimal notation of IP addresses. This reversed ordering of the domains for each octet value is needed because, unlike DNS names, when IP addresses are read from left to right, they are interpreted in the opposite manner.
When an IP address is read from left to right, it is viewed from its most generalized information (an IP network address) in the first part of the address to the more specific information (an IP host address) contained in the last octets. For this reason, the order of IP address octets must be reversed when building the in-addr.arpa domain tree. The IP addresses of the DNS in-addr.arpa tree can be delegated to companies as they are assigned a specific or limited set of IP addresses within the Internet-defined address classes. Finally, the in-addr.arpa domain tree, as built into DNS, requires that an additional resource record (RR) type the pointer (PTR) RR be defined. This RR is used to create a mapping in the reverse lookup zone that typically corresponds to a host (A) named RR for the DNS computer name of a host in its forward lookup zone.
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