The correct answer is variable-length subnet mask (VLSM). This technique enables network administrators to divide a network into subnets of different sizes. Instead of wasting address space by using the same mask throughout the entire network, VLSM allows subnets to be tailored according to the number of hosts needed, which conserves addresses and makes more efficient use of the address space. Other options either do not relate to subnet size variation or serve a completely different function in networking.