From Paterva Wiki
This transform returns a netblock (IP range) by simply looking at the natural network boundary of the IP address. The size of the network is determined by a transform setting ('Block size'). The size is set by default to 256 - meaning that the corresponding class C network will be returned. This size can be set to any power of two - e.g. 1,2,4,8,16,32,64,128,256 etc. As this transform is not doing any look ups it is very fast and by setting the block size small (making some assumptions) you can quickly get a rough idea of networks involved.