From Paterva Wiki
This transform works on [[NSrecord[[s. It determines if the particular name server has any Netblock reverse DNS delegated to it. This is useful for finding Netblock of an organisation. What's interesting about the results of this transform is that an organisation might have a class B network (a fairly large Netblock), but, in reality are only using a couple of class Cs (smaller netblocks) within that block. In many cases they will only populate the reverse DNS of these smaller blocks and delegate it to their name servers. The transform will show these smaller blocks.
NSrecord via transform NSrecordToNetblock_NS4block to