Review of CIDR and VLSM

VLSM

Another way to view the VLSM subnets is to list each subnet and its sub-subnets.

In Figure 1, the 10.0.0.0/8 network is the starting address space and is subnetted with a /16 mask. Borrowing 8 bits (going from /8 to /16) creates 256 subnets that range from 10.0.0.0/16 to 10.255.0.0/16.

In Figure 2, the 10.1.0.0/16 subnet is further subnetted by borrowing 8 more bits. This creates 256 subnets with a /24 mask. This mask allows 254 host addresses per subnet. The subnets ranging from 10.1.0.0/24 to 10.1.255.0/24 are subnets of the subnet 10.1.0.0/16.

In Figure 3, the 10.2.0.0/16 subnet is also further subnetted with a /24 mask allowing 254 host addresses per subnet. The subnets ranging from 10.2.0.0/24 to 10.2.255.0/24 are subnets of the subnet 10.2.0.0/16.

In Figure 4, the 10.3.0.0/16 subnet is further subnetted with a /28 mask, thus creating 4,096 subnets and allowing 14 host addresses per subnet. The subnets ranging from 10.3.0.0/28 to 10.3.255.240/28 are subnets of the subnet 10.3.0.0/16.

In Figure 5, the 10.4.0.0/16 subnet is further subnetted with a /20 mask, thus creating 16 subnets and allowing 4,094 host addresses per subnet. The subnets ranging from 10.4.0.0/20 to 10.4.240.0/20 are subnets of the subnet 10.4.0.0/16. These /20 subnets are big enough to subnet even further, allowing more networks.