Careful consideration must be given to the design of a network addressing scheme. For example, the sample topology in Figure 1 requires seven subnets.
Using traditional subnetting, the first seven address blocks are allocated for LANs and WANs, as shown in Figure 2. This scheme results in 8 subnets with 30 usable addresses each (/27). While this scheme works for the LAN segments, there are many wasted addresses in the WAN segments.
If an addressing scheme is designed for a new network, the address blocks can be assigned in a way that minimizes waste and keeps unused blocks of addresses contiguous. It can be more difficult to do this when adding to an existing network.
As shown in Figure 3, to use the address space more efficiently, /30 subnets are created for WAN links. To keep the unused blocks of addresses together, the last /27 subnet is further subnetted to create the /30 subnets. The first 3 subnets were assigned to WAN links creating subnets 192.168.20.224/30, 192.168.20.228/30, and 192.168.20.232/30. Designing the addressing scheme in this way leaves three unused /27 subnets and five unused /30 subnets.
Figures 4 to 7 display sample configurations on all four routers to implement the VLSM addressing scheme.