zone "16/20.168.192.in-arpa.arpa" {
        type master;
        file "/path/to/a/new/16.168.192.in-arpa.arpa-db-file";
        allow-update {  key copy-this-from-the-other-reverse-zone; };
    }

$TTL 2d ; 172800
    $ORIGIN 64/27.23.168.192.IN-ADDR.ARPA.
    @            IN  SOA   ns1.example.com. hostmaster.example.com. (
                          2003080800 ; serial number
                          3h         ; refresh
                          15m        ; update retry
                          3w         ; expiry
                          3h      ; nx = nxdomain ttl
                          )
          IN  NS      ns1.example.com.
          IN  NS      ns2.example.com.