Why are in-addr.arpa addresses backwards?
An "in-addr.arpa address" is a reverse DNS record, stored in a strange format. If we are
considering ip 184.108.40.206, then "220.127.116.11.in-addr.arpa" is the reverse
DNS record. It's used when you want to find out the host name of
something you have an ip address for (for example, "dig -x " will
give you that). But why is it stored backwards?
Well, if you wanted to get really geeky about it, it's the ip
addresses that you are used to seeing that are backwards. Think
about it this way: when you see mail.xyz.com, what's the least
specific part of that? It's the ".com", because there are millions
of ".com" machines. The "xyz.com" may have multiple machines, but
"mail.zyz.com" nails it down to one.
Well, maybe more than one, because
of round-robin DNS, or multiple MX records, but that's not
important for this discussion.
Now look at the ip address -
let's pretend that it's 192.168.4.5. What's the least specific part
of that? The "192", of course. So in names, the least specific part
comes last, but in IP addresses, it comes first. Therefore, from a
geek point of view, at least one of them is "backwards".
When DNS is used to find something on the internet, it always
starts at the least specific. We ask ".com" to tell us who is
responsible for "xyz.com" and we ask that DNS server where to find
"mail". Likewise, if we are working from an address to find a name,
we want to work the same way: go to the least specific first. Ask
the machine responsible for 192 addresses to tell us where 192.168
is, etc. (except, of course, that 192.168 is one of the private
address ranges so that doesn't really work) That's why in-addr.arpa
records are stored in the same way as the names are: most specific
Of course if the convention for addresses had been to present
them like we present names, then you'd say " This machine is
18.104.22.168", and the in-addr.arpa address might be what we
are used to seeing..
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© 2011-07-06 Tony Lawrence