It is common practice to ensure both name to address, and address to name mappings are provided for networks. This practice, while documented, has never been documented as a requirement placed upon those who control address blocks. This document fills this gap. As we can see, the regional registries have their own policies for requirements for IN-ADDR maintenance. It should be noted, however, that many address blocks were allocated before the creation of the regional registries, and thus it is unclear whether any of the policies of the registries are binding on those who hold blocks from that era.
Many applications use DNS lookups for security checks.
To ensure validity of claimed names, some applications will look up IN-ADDR records to get names, and then look up the resultant name to see if it maps back to the address originally known. Failure to resolve matching names is seen as a potential security concern. Requirements All IP address space which is assigned and in use MUST be resolved by IN-ADDR records. Internet providers and other users to whom a block of addresses are delegated MUST provide for lookup of host names from IP addresses. This may be provided directly or by delegation to the user of the address block.