To generate PTR reverse records with $GENERATE, we can do so
$GENERATE 1-255 $ PTR 168-121-40-$.isp.domain.net.
but now how to generate A record of type
168-121-40-1.isp IN A 168.121.40.1
168-121.40.2.isp IN A 168.121.40.2
...
168-121.40.255.isp IN A 168.121.40.255