With public networks, the address savings help engineers avoid having to obtain another registered IP network number from regional IP address assignment authorities. VLSM can be helpful for both public and private IP addresses, but the benefits are more dramatic with public networks. By wasting fewer addresses, more space remains to allocate more subnets.
This flexibility reduces the number of wasted IP addresses in each subnet.
For example, for subnets that need fewer addresses, the engineer uses a mask with fewer host bits, so the subnet has fewer host IP addresses. Because a mask defines the size of the subnet (the number of host addresses in the subnet), VLSM allows engineers to better match the need for addresses with the size of the subnet. VLSM provides many benefits for real networks, mainly related to how you allocate and use your IP address space. In that case, the design does not use VLSM. However, Class A network 10.0.0.0 uses only one mask, and Class A network 11.0.0.0 uses only one mask. Oddly enough, a common mistake occurs when people think that VLSM means “using more than one mask in some internetwork” rather than “using more than one mask in a single classful network.” For example, if in one internetwork diagram, all subnets of network 10.0.0.0 use a 255.255.240.0 mask, and all subnets of network 11.0.0.0 use a 255.255.255.0 mask, the design uses two different masks.
All subnets are of Class A network 10.0.0.0, with two masks being used, therefore meeting the definition of VLSM.
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