IP subnetting seems to trip up quite a few people in the networking world. I've known experienced consultants who have worked in the industry for 15 years and still resort to subnet calculators.
IP subnetting can be a daunting subject for those who are not familiar with it. The principles of subnetting are based on binary (and some mathematical) principles such as eXclusive-OR (XOR), which, for many people, are foreign concepts or aspects of college courses long-since forgotten.
There are really only two times when your average networking people need to know how subnet math truly works: when they study for their first networking certification (the CCNA), and when they study for their last (the CCIE). In fact, I don't really think the math is needed for the CCNA coursework, although Cisco makes you learn it. The CCIE exam does make you do bizarre things with subnet masks that require a full understanding of the math behind the topic. However, these things are rarely, if ever, seen in the real world, and are not a topic for this book.
If you want to be able to do IP subnetting in your head, there are a couple of things you will need to understand. First, Cisco, along with every other major manufacturer out there, wants you to learn subnetting its way. Why? So its tests are harder to pass, and so everyone who's certified speaks the same language.
Cisco, in particular, seems to want people to think in a way that makes it harder to figure out what's really going on. ...