This is a database file that is used for reverse-lookups in particular IP DNS zones of host names when supplied with the IP numbers. This allows a resolver to provide an IP address and request a matching host name. This file contains SOA and name server records similar to other DNS database zone files. It also contains pointer records.
This DNS reverse-lookup capability is important because some applications provide the capabilities to implement security based on the connecting host names. For instance, if a client tries to link to a Network File System (NFS) volume with this security arrangement, the NFS server would contact the DNS server and do a reverse-name lookup on the clients IP address. If the host name returned by the DNS server is not in the access list for the NFS volume or if the host name was not found by DNS, then the NFS mount request would be denied. This reverse-lookup capability is often used for troubleshooting reasons as well, which will be discussed later in this document.
Here are a couple example zones for different IP class networks.
Example class C zone:
100.200.192.in-addr.arpa
Example class B zone:
55.157.in-addr.arpa
Pointer records provide a static mapping of IP addresses to host names within a reverse-lookup zone. IP numbers are written in backward order and "in-addr.arpa." is appended to the end to create this pointer record. As an example, looking up the name for "157.55.200.51" requires a PTR query for the name "51.200.55.157.in-addr.arpa."
<ip reverse domain name> IN PTR <host name>
Example:
51.200.55.157.in-addr.arpa. IN PTR mailserver1.microsoft.com.