durance pri # cat 0.168.192.zone 
$TTL    86400

@       IN      SOA     dns.domain.com. root.dns.domain.com. (
                3       ;Serial
                8H      ;Refresh 8 Hours
                2H      ;retry 2 Hours
                1W      ;Expire 1 Week
                1D      ;Minimum 1 Day
        )
        NS      dns.domain.com.

21      PTR     durance.domain.com.
        PTR     dns.domain.com.
        PTR     mail.domain.com.
20      PTR     collapse.domain.com.