28/28 192.168.1.144/28 192.168.1.144/28 192.168.1.144/28 192.168.1.144/28 192.168.1.144/28 192.168.1.144/28 192.168.1.144/28 192.168.1.144/28 192.168.1.160/28 192.168.1.160/28 192.168.1.160/28 192.168.1.160/28 192.168.1.160/28 192.168.1.160/28 192.168.1.160/27 192.168.1.160/27 192.168.1.176/28 192.168.1.176/28 192.168.1.176/28 192.168.1.176/28 192.168.1.176/28 192.168.1.176/28 192.168.1.192/28 192.168.1.192/28 192.168.1.192/28 192.168.1.192/28 192.168.1.192/28 192.168.1.192/28 192.168.1.192/28 192.168.1.208/28 192.168.1.208/28 192.168.1.208/28 192.168.1.208/28 192.168.1.208/28 192.168.1.208/28 192.168.1.208/28 192.168.1.192/26 192.168.1.224/28 192.168.1.224/28 192.168.1.224/28 192.168.1.224/28 192.168.1.224/28 192.168.1.224/27 192.168.1.224/27 192.168.1.240/28 192.168.1.240/28 192.168.1.240/28 192.168.1.240/28 192.168.1.240/28 Figure 34-7. Divide-by-half IP subnet allocation Chapter 34: IP Design Reverse Binary Reverse binary is Cisco's recommended method for IP subnet allocation. In this method, subnets are allocated by counting in binary, but with the most and least sig- nificant bits reversed. Figure 34-8 shows how reverse binary works. The normal binary representations of the digits 0-7 are shown on the left. By reversing the signif- icant bit order, we create a mirror image of the numbers in binary (shown on the right). These numbers happen to correspond to the subnets that should be used, in order, regardless of size. The pattern may be continued for as long as is useful. Normal binary Reverse binary 00000000 00000000 00000001 10000000 00000010 01000000 00000011 11000000 00000100 00100000 00000101 10100000 00000110 01100000 00000111 11100000 Figure 34-8. Reverse-binary subnet allocation By allocating subnets in this order, we end up spacing the subnets SO that no two subnets are adjacent unless the subnets are of varying sizes (a distinct possibility using VLSM). This method also offers a reasonable tradeoff of allowing the largest possible remaining addressable space while offering the expandability desire