An organization has been assigned the prefix 173.215.64.0/21. It wishes to subdivide this address range, establishing four subnets within the organization. Subnet A must be able to accommodate at least 1000 interfaces, subnet B at least 500, and subnets C and D at least 200 each. What would suitable prefixes, expressed in a.b.c.d/x form, be for the four subnets? Use Variable length Subnet Mask.