2/28 192.168.1.32/27 192.168.1.32/27 192.168.1.48/28 192.168.1.48/28 192.168.1.48/28 192.168.1.48/28 192.168.1.48/28 192.168.1.48/28 192.168.1.64/28 192.168.1.64/28 192.168.1.64/28 192.168.1.64/28 192.168.1.64/27 192.168.1.80/28 192.168.1.80/28 192.168.1.80/28 192.168.1.80/28 192.168.1.64/26 192.168.1.64/26 192.168.1.64/26 192.168.1.96/28 192.168.1.96/28 192.168.1.96/27 192.168.1.96/27 192.168.1.96/27 192.168.1.112/28 192.168.1.112/28 192.168.1.128/25 192.168.1.128/25 192.168.1.128/25 192.168.1.128/25 192.168.1.128/25 192.168.1.128/25 192.168.1.128/25 192.168.1.128/25 Figure 36-6. Sequential IP subnet allocation Allocating IP Subnets In the first edition of this book, I stated that the broadcast network was unusable. Old habits die hard, especially for old nerds like me. It still pains me to use the last network when designing networks, but that's more an indication that I've been doing this for far too long than anything else. Cisco documentation (http://www.cisco.comlen/US/ tech/tk648/tk361/technologies_tech_note09186a0080093f18.shtml) states: On the issue of using subnet zero and the all-ones subnet, RFC 1878 states, "This practice (of excluding all-zeros and all-ones subnets) is obsolete. Modern software will be able to utilize all definable networks." Today, the use of subnet zero and the all-ones subnet is generally accepted and most vendors support their use. However, on certain networks, particularly the ones using legacy software, the use of subnet zero and the all-ones subnet can lead to problems. 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 36-7 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