PTR (Pointer) Record

Unlike the other Start of Authority (SOA) records, PTR records are used only in reverse (IN-ADDR.ARPA) domains. There must be exactly one PTR record for each Internet address. For example, if the host gadzooks.poetry.arizona.edu has an IP address of 128.196.47.55, then there must be a PTR record for it in the following format:

55.47.196.128.IN-ADDR.ARPA.  IN   PTR  gadzooks.poetry.arizona.edu.
          

Reverse domains contain mainly PTR records (plus SOA and NS records at the top.)

The Berkeley r-utilities use the value of the PTR record for hostname authentication. Although the Domain Name System (DNS) specifies that case is not significant in host names, be aware that some operating systems are sensitive to host name case.