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 1.2.3.4, then "4.3.2.1.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 to least.

Of course if the convention for addresses had been to present them like we present names, then you'd say " This machine is 34.2.168.192", and the in-addr.arpa address might be what we are used to seeing..



Got something to add? Send me email.





(OLDER) <- More Stuff -> (NEWER)    (NEWEST)   

Printer Friendly Version

-> -> Why are in-addr.arpa addresses backwards?


1 comment



Increase ad revenue 50-250% with Ezoic


More Articles by

Find me on Google+

© Tony Lawrence







Fri Feb 25 15:11:30 2005: 72   anonymous


Very very well explaned thank you ever so much.

------------------------
Kerio Samepage


Have you tried Searching this site?

Unix/Linux/Mac OS X support by phone, email or on-site: Support Rates

This is a Unix/Linux resource website. It contains technical articles about Unix, Linux and general computing related subjects, opinion, news, help files, how-to's, tutorials and more.

Contact us





Today, kernels are too much obedient servants, blindly doing the bidding of any program that asks. (Tony Lawrence)

Random numbers should not be generated with a method chosen at random (Donald Knuth)







This post tagged: