Finding the Pattern Using the Magic Number
Subnet IDs follow a predictable pattern, at least when using our assumption of a single subnet mask for all subnets of a network. The pattern uses the magic number, as discussed in Chapter 14, “Analyzing Existing Subnets.” To review, the magic number is 256, minus the mask’s decimal value, in a particular octet that this book refers to as the interesting octet.
Figure 19-7 shows four examples of these patterns with four different masks. For example, just look at the top of the figure to start. It lists mask 255.255.128.0 on the left. The third octet is the interesting octet, with a mask value other than 0 or 255 in that octet. The left side shows a magic number calculated as 256 – 128 = 128. So, the ...
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