56*4 32*256*256*256 256*8 32*256*256 256*256*8 32*256 256 * 256 * 256 * 8 16*256*256*256 256*16 16*256*256 256*256*16 16*256 256*256*256*16 8*256*256*256 256*32 8*256*256 256*256*32 8*256 256*256*256*32 4*256*256*256 256*64 4*256*256 256*256*64 4*256 256*256*256*64 2*256*256*256 256*128 2*256*256 256*256*128 2*256 256 * 256 * 256 * 128 1*256*256*256 256*256 1*256*256 256*256*256 1*256 256*256*256*256 Figure 34-12. Subnet octet ratios Using the class C network 192.168.1.0 255.255.255.0 as an example, if you apply a subnet mask of 255.255.255.224, the result is 8 subnets with 32 hosts in each. Look at Figure 34-11, and you'll see that the ratio for the subnet octet 224 is 8:32. The ratios happen to correlate with the number of subnets and hosts per subnet in a native class C network. If you look at the other columns, you'll notice that the ratio is the same, but it is in a different position in the equation. Let's look at a single example. Figure 34-13 shows the ratios for the subnet octet of 224. First octet Second octet Third octet Fourth octet 32*256*256*256 256* 8 32 * 256 * 256 256*256*8 : 32 * 256 256*256*256*8 Figure 34-13. 224 ratios In practice, here's what the chart is saying, using the network 10.0.0.0: 10.0.0.0 224.0.0.0 = 8 subnets of 536,870,912 (32 * 256 256 256) hosts 10.0.0.0255.224.0.0 = 2,048 (256 * 8) subnets of 2,097,156 (32 256 * 256) hosts 0.0.0.0 255.255.224.0 = 524,288 (256 * 256 8) subnets of 8,192 (32 * 256) hosts 10.0.0.0255.255.255.224 = 134,217,728 (256 * 256*256 8) subnets of 32 hosts Notice that in each case, somewhere in the equation, the ratio of 8:32 appears. IP Subnetting Made Easy That's all well and good, but it still looks like a lot of icky math to me, SO lets make it even simpler. This leads us to our third rule: When you know these rules, all you need to be able to do is to double or halve numbers. Subnet masks, like all things computer-related, are based on binary. Because binary is based on powers of two, all you really need to be able to do is to double a number, or cut 