IpAddressPoolSubnet can either be carved out of a PolicyBlock or statically specified by the user. In the first case where the subnet is carved out of a IpAddressBlock, the user must specify the ID of the block from which this subnet is to be derived. This block must be pre-created. The subnet range is auto populated by the system. In the second case, the user configures the subnet range directly. No IpAddressBlock is required.