Reverse Zones

When a computer is contacted by another computer, the IP stack can supply only the IP number of the originating computer. If the destination computer needs to know the name of the originating computer, for whatever reason, it must find a way to get this information. This also is DNS's job.

To make a reverse lookup possible, you must be able to look up the address and get a name back. In IPv4, this is done with a rather straightforward mechanism. If you have the address of a peer host on the network, such as 129.240.222.66, you look up the name 66.22.240.129. in-addr.arpa in DNS and get the name gilgamesj.uio.no back.

The name is simply the IP address with each address octet in reverse order, with the domain name .in-addr.arpa appended. The in-addr.arpa domain is a special domain that is used only for reverse lookups. The in-addr part stands for Internet address, and the arpa comes from the old name of the Internet, the ARPAnet. Therefore, you look up the internet addresses of the ARPAnet in it.

The reverse order of the IP number is a nuisance to write out, and it would also be easier to read it if the IP number was not reversed. However, that would not work. Because IP numbers are also hierarchic, but from left to right, the octets need to be reversed so they are hierarchic from right to left, just as DNS is. Other than that, the in-addr.arpa domain is, technically, just like the other, more familiar, forward domains.