tered three methods for allocating IP subnets. I'll call these methods se- quential, divide by half, and reverse binary. Sequential Most people's first inclination seems to be to allocate IP addresses and IP subnets in numerical order. That is, they allocate the first numerical subnet, then the next, and SO on. If such a person were subnetting the 192.168.100.0/24 network into /30 subnets, Chapter 36: IP Design she would likely allocate them in this order: 0, 4, 8, 12, etc. Sequential allocation of subnets in a network is what most admins seem naturally prone to do. It works, it's easy to understand, and it's pretty easy to tell where the next available network will be. Of the three methods I'll discuss here, this is the least desirable and the most often deployed. There are some serious problems with this method. First, there is no room for expansion in any of the subnets, except for possibly the last one used. If the subnet 192.168.100.16 grows and now needs 18 addresses instead of 16, the entire subnet must be renumbered to a larger subnet space elsewhere. Second, this method does not allocate addresses in a manner that allows for the largest available block to be concur- rent and addressable. Another problem is future availability of subnets. Remember, with VLSM, you are not limited to having every subnet be the same size. If you plan carefully, you can use your given space very efficiently. Figure 36-6 shows how we might sequentially allocate a series of /28 subnets within the 192.168.1.0/24 network. Each time we need another /28 subnet, we allocate the next one available. I've broken out the subnets to show the largest possible network space remaining after each allocation. 192.168.1.0/28 192.168.1.0/28 192.168.1.0/28 192.168.1.0/28 192.168.1.0/28 192.168.1.0/28 192.168.1.0/28 192.168.1.0/28 192.168.1.16/28 192.168.1.16/28 192.168.1.16/28 192.168.1.16/28 192.168.1.16/28 192.168.1.16/28 192.168.1.16/28 192.168.1.16/28 192.168.1.32/28 192.168.1.32/28 192.168.1.32/28 192.168.1.32/28 192.168.1.32/28 192.168.1.3