CCENT/CCNA ICND1 100-105 Official Certification Guide, First Edition

Appendix N. Classless Inter-domain Routing

The chapters in Part IV of the book work through the fundamentals of IPv4 addressing. However, most of the discussion about starting with a big block of addresses, and then creating smaller blocks called subnets, begins with a classful network (that is, a Class A, B, or C network). A thorough understanding of how to take a classful network and subdivide it into subnets is very important, because most enterprises do exactly that: They start with some private IPv4 network, often network 10.0.0.0, and divide it into subnets.

However, most enterprises also use Classless Inter-domain Routing (CIDR) with regard to the public IPv4 addresses used by the company. CIDR defines many ideas, including how an enterprise ...