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The TCP/IP Guide  9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)       9  Name Systems and TCP/IP Name Registration and Name Resolution            9  TCP/IP Name Systems: Host Tables and Domain Name System (DNS)                 9  TCP/IP Domain Name System (DNS)                      9  DNS Name Servers and Name Resolution                           9  DNS Resolution Concepts and Resolver Operations

DNS Name Resolution Process 1 2 3 4 DNS Electronic Mail Support and Mail Exchange (MX) Resource Records

Why The IN-ADDR.ARPA Structure Is the Opposite of Normal IP Addresses

I'm sure you've noticed that the numbers are “backwards” in the IN-ADDR.ARPA domain. We've already seen the reason for this: name resolution proceeds from the least-specific to the most-specific element, going from right to left. In contrast, IP addresses have the least specific octet on the left, and the most specific on the right. Thus, we reverse them to maintain consistency with the DNS name space.

This immediately yields us one extra benefit. Just as we can delegate authority for portions of the regular name space, for example, letting “XYZ Industries” be in charge of everything in “xyzindustries.com”, we can also delegate authority for parts of the IN-ADDR.ARPA name space. For example, since MIT owns all IP addresses with a first octet of 18—at least I think they still do—it is possible that if MIT wanted to, they could control the “18.IN-ADDR.ARPA” domain as well for reverse queries. This would not be possible without reversing the octets as we have done.

Note that for this system to work reliably, it is essential that the data in the “regular” name space and the reverse name space remain consistent. Whenever a new DNS name is registered, an appropriate entry must be made within IN-ADDR.ARPA as well. Special procedures have been put into place to allow these pointer entries to be created automatically.

DNS Name Resolution Process 1 2 3 4 DNS Electronic Mail Support and Mail Exchange (MX) Resource Records