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

Chapter 45
Classes A–D Review

For most of us, Classes A through D is what we will be working with. We will never have to dabble in the classless society of addresses. Classes A through D will be around for a long time and IPv6, although adopted, is still quite a few years away. IPv6 does not understand the concept of class networking and supplies enough addresses for millions of years to come—enough to supply an IP address for all those refrigerators and washers and dryers. (Don’t laugh, this will happen. Why? Think of maintenance, or being able to control things in your house via your browser. Forget to turn off some lights or set the security up? The possibilities are endless.)

For those newbies, the easiest way to remember IP class addresses is this: The first byte will always identify the class address. Whether you have converted to binary or are looking at the address in its dotted decimal form, the first byte gives it away. A is the first letter in the alphabet, and therefore a Class A network address is only the first byte, leaving the last three fields for host addressing. B is the second letter in the alphabet, and therefore the network portion of the address is the first 2 bytes of the address, leaving the last two fields for host address. C is the third letter in the alphabet, and the network portion takes up the first 3 bytes of the address and leaves one field for host addresses. As for remembering which number is associated to which class, the only field that is important is the first field. Memorize the starting network number for each class.

The classes are

Class A: 0–127

Class B: 128–191

Class C: 192–223

Class D: 223–239

Reserved: 240–254