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
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