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DNS Reverse Name Resolution Using the IN-ADDR.ARPA Domain
(Page 2 of 4)
The IN-ADDR.ARPA Name Structure for Reverse Resolution
The problem here is that the servers
are arranged by name and not be IP address. The solution, therefore,
is as simple as it sounds: arrange the servers by IP address. This doesn't
mean we remove the name hierarchy, or duplicate all the servers, or
anything silly like that. Instead, we create an additional, numerical
hierarchy that coexists with the name hierarchy. We then use this to
find names from numbers, using a process commonly called reverse
The name hierarchy for the Internet
is implemented using a special domain called IN-ADDR.ARPA,
located within the reserved .ARPA
top-level domain (IN-ADDR
stands for INternet ADDRess. Recall that .ARPA
was originally used to transition old Internet hosts to DNS, and is
now used by the folks that run the Internet for various purposes.
A special numerical hierarchy is
created within IN-ADDR.ARPA that covers the entire IP address
- At the first level within IN-ADDR.ARPA
there are 256 subdomains called 0, 1, 2
and so on, up to 255. For example, 191.IN-ADDR.ARPA.
(Actually there may not be all 256 of these since some IP addresses
are reserved, but let's ignore that for now).
- Within each of the subdomains above, there are
256 further subdomains at the second level, numbered the same way. So
for example, one of these would be 27.191.IN-ADDR.ARPA.
- Again, there are 256 subdomains at the third
level within each of the above, such as 203.27.191.IN-ADDR.ARPA
- Finally, there are 256 subdomains at the fourth
level within each of the third-level subdomains, such as 220.127.116.11.IN-ADDR.ARPA.
This structure is illustrated in
As you can see, within IN-ADDR.ARPA we have created a name
space that parallels the address space of the Internet Protocol. (Yes,
this means there are several billion nodes and branches
in this part of the Internet DNS name space!)
Figure 246: The DNS IN-ADDR.ARPA Reverse Name Resolution Hierarchy
The special IN-ADDR.ARPA hierarchy was created to allow easy reverse lookups of DNS names. IN-ADDR.ARPA contains 256 subdomains numbered 0 to 255, each of which has 256 subdomains numbered 0 to 255, and so forth, down four levels. Thus, each IP address is represented in the hierarchy. In this diagram I have shown as an example the DNS domain name www.xyzindustries.com. It would have a conventional resource record pointing to its IP address, 18.104.22.168, as well as a reverse resolution record at 22.214.171.124.IN-ADDR.ARPA, pointing to the domain name www.xyzindustries.com.
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Version 3.0 - Version Date: September 20, 2005
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