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12.3.4. Reverse Name Resolution Zone Files

A reverse name resolution zone file is used to translate an IP address in a particular namespace into a FQDN. It looks very similar to a standard zone file, except that PTR resource records are used to link the IP addresses to a fully qualified domain name.
A PTR record looks similar to this:
<last-IP-digit>      IN     PTR    <FQDN-of-system>
The <last-IP-digit> is the last number in an IP address which points to a particular system's FQDN.
In the follow example, IP addresses through are pointed to corresponding FQDNs.
$TTL 86400
@     IN     SOA (
                    2001062501 ; serial
                    21600      ; refresh after 6 hours
                    3600       ; retry after 1 hour
                    604800     ; expire after 1 week
                    86400 )    ; minimum TTL of 1 day

      IN     NS
      IN     NS

20    IN     PTR
21    IN     PTR
22    IN     PTR
23    IN     PTR
24    IN     PTR
25    IN     PTR
This zone file would be called into service with a zone statement in the named.conf file which looks similar to the following:
zone "" IN {
  type master;
  file "";
  allow-update { none; };
There is very little difference between this example and a standard zone statement, except for the zone name. Note that a reverse name resolution zone requires the first three blocks of the IP address reversed followed by This allows the single block of IP numbers used in the reverse name resolution zone file to be associated with the zone.