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DNS Reverse Name Resolution Using the IN-ADDR.ARPA Domain
(Page 4 of 4)
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.
 Related Information: A similar scheme using a different reverse domain is used for DNS under IPv6.
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 Key Concept: Most name resolutions require that we transform a DNS domain name into an IP address. However, there are cases where we want to perform a reverse name resolution, by starting with an IP address and finding out what domain name matches it. This is difficult to do using the conventional DNS distributed name hierarchy, because there is no easy way to find the DNS server containing the entries for a particular IP address using the regular DNS name hierarchy. To this end, a special hierarchy called IN-ADDR.ARPA was set up for reverse name lookups. This hierarchy contains four levels of numerical subdomains structured so that each IP address has its own node. The node for an IP address contains an entry that points to the DNS domain name associated with that address.
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