Pointer (PTR) Resource Records

Pointer records (PTR) look like this:

   190.18.204.198   IN   PTR   www.zippo.net.
   1.18.204.198     IN   PTR   cerebrus.zippo.net. 

Note that we are trying to resolve 198.204.18.190 back to www.zippo.net. PTR records list the IP addresses in reverse order, from most specific to least specific, just as names are done. When you think about it, it makes sense. The most specific part of a fully qualified host name is on the left as is the most specific part of the IP address if you turn it around. In the fully qualified host name www.zippo.net, the host name www is the most specific part, and it's on the left. In the IP address 198.204.18.190, 190 is the most specific part but it's on the RIGHT, so we have to reverse the order of the octets to put it on the left. This way the software which does forward resolution can do reverse resolution the same way. Have a look at reverse resolution if you haven't done so already.