Beside looking up the IP address belonging to a host, it is sometimes desirable to find out the canonical host name corresponding to an address. This is called reverse mapping and is used by several network services to verify a client's identity. When using a single hosts file, reverse lookups simply involve searching the file for a host that owns the IP address in question. With DNS, an exhaustive search of the name space is out of the question, of course. Instead, a special domain, in-addr.arpa, has been created which contains the IP addresses of all hosts in a reverted dotted-quad notation. For instance, an IP address of 22.214.171.124 corresponds to the name 126.96.36.199.in-addr.arpa. The resource record type linking these names to their canonical host names is PTR.
Creating a zone of authority usually means that its administrators are given full control over how they assign addresses to names. Since they usually have one or more IP networks or subnets at their hands, there's a one-to-many mapping between DNS zones and IP networks. The Physics Department, for instance, comprises the subnets 188.8.131.52, 184.108.40.206, and 220.127.116.11.
As a consequence, new zones in the in-addr.arpa domain have to be created along with the physics zone and delegated to the network administrators at the department: 8.76.149.in-addr.arpa, 12.76.149.in-addr.arpa, and 14.76.149.in-addr.arpa. Otherwise, installing a new host at the Collider Lab would require them to contact their parent domain to have the new address entered into their in-addr.arpa zone file.
The zone database for subnet 12 is shown in figure . The corresponding glue records in the database of their parent zone is shown in figure .
Figure: An excerpt from the named.rev file for subnet 12.
Figure: An excerpt from the named.rev file for network 149.76.
One important consequence of this is that zones can only be created as supersets of IP networks, and, even more severe, that these network's netmasks have to be on byte boundaries. All subnets at Groucho Marx University have a netmask of 255.255.255.0, whence an in-addr.arpa zone could be created for each subnet. However, if the netmask was 255.255.255.128 instead, creating zones for the subnet 18.104.22.168 would be impossible, because there's no way to tell DNS that the 12.76.149.in-addr.arpa domain has been split in two zones of authority, with host names ranging from 1 through 127, and 128 through 255, respectively.