Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 46
Subnetting

Now that IP address assignment has been shown, let’s further confuse the issue by looking at subnet masks. Another name for subnet masks is extended network prefix. This book will continue to use the well–known name of subnetting. Subnetting is explained in RFC 950.

Implementing classes in network numbers gave us some hierarchical structure to the Internet. Using class assignment, you could select a network number based on the number of hosts that are on or will be on your network. But the range was very limited. Class A gave you a lot of hosts but just a few networks. Class B was the one picked to allow for a balance of hosts and networks, and Class C allowed many networks and a few hosts. Not much choice, either you had a lot of networks or a lot of hosts. The most requested network number was Class B; however, many Class B assignments were not fully used—really hard to have 65,535 hosts on a single network. Too many Class C addresses filled up routing tables and most did not fully use all 254 host addresses. Furthermore, some sites were requesting multiple addresses to fulfill their needs.

Not many Class A addresses were handed out. In fact, after about 63 assignments, Class A assignments were not handed out at all. Class B addresses were popular and were the most frequently asked for address class. What’s the deal with Class C addresses? With only 254 hosts available for assignment, many Class C addresses have to be assigned. Again, using Class assignment, the routing tables started to fill up and most of the bits were wasted when implemented. It was like being given a five–passenger car, but you never had anyone in the other seats. In short, subnetting allows for tremendous efficiency not only in Internet routing tables but also on customer networks as well. It allows us to assign some of the bits normally used by the host portion of the address and reassign these bits to the network portion of the address. This is accomplished for the reasons that follow.

Subnetting