192.168.1.80/28 192.168.1.80/28 192.168.1.64/26 192.168.1.64/26 192.168.1.64/26 192.168.1.96/28 192.168.1.96/28 192.168.1.96/27 192.168.1.96/27 192.168.1.96/27 192.168.1.112/28 192.168.1.112/28 192.168.1.128/25 192.168.1.128/25 192.168.1.128/25 192.168.1.128/25 192.168.1.128/25 192.168.1.128/25 192.168.1.128/25 192.168.1.128/25 Figure 34-6. Sequential IP subnet allocation The diagrams in these sections are keyed as follows: the lightest sub- nets have not been allocated, the middle-shaded subnets have been allocated, and the darkest subnets are not usable (i.e., are broadcast subnets). Allocating IP Subnets Divide by Half This method prescribes that every time a new network is allocated, the smallest avail- able chunk of addresses is divided by half. The idea here is to maintain the largest possible block of addressable space. To that end, we allocate the middle subnet first, thus leaving the large /25 block available. Figure 34-7 shows the divide-by-half method in action. In this example, I've taken the same 192.168.1.0/24 network, and allocated /28 subnets accordingly. Using this method allows for each subnet to be expanded, as there is space after each used subnet equal to the size of the subnet in use. This method also allows for the largest possible subnet to be available at any given time. I like this method because it rea- sonably balances the need for subnet expandability while keeping the largest free space available for as long as possible. 192.168.1.0/28 192.168.1.0/27 192.168.1.16/28 192.168.1.0/26 192.168.1.0/26 192.168.1.32/28 192.168.1.32/28 192.168.1.48/28 192.168.1.48/28 192.168.1.0/25 192.168.1.0/25 192.168.1.0/25 192.168.1.0/25 192.168.1.64/28 192.168.1.64/28 192.168.1.64/28 192.168.1.64/28 192.168.1.80/28 192.168.1.80/28 192.168.1.80/28 192.168.1.80/28 192.168.1.96/28 192.168.1.96/28 192.168.1.96/28 192.168.1.96/27 192.168.1.112/28 192.168.1.112/28 192.168.1.112/28 192.168.1.128/28 192.168.1.128/28 192.168.1.128/28 192.168.1.128/28 192.168.1.128/28 192.168.1.128/28 192.168.1.128/28 192.168.1.1