there is 2 kind of dns zone, forward and reverse.
Normally in our live we use forward lookup, means inside a zone test.com we will map www -> ip1, smtp ->ip2, pop3 -> ip3 and etc. All host name map to particular ipaddress via 'A' record.
PTR use for reverse lookup zone, allow us to map ip address to difference host. the pattern can be 0.168.192, then 1-> www , 2-> smtp, 3->pop3. The map ing between last octet of ipaddress is using PTR.
So, the same zone in reverse lookup actually is same subnet.