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

Chapter 40
Identifying a Class

For network and host assignment, Classes A through C are used. Class D is not used for this, and Class E is never assigned. Referring to the slide, we can see how the classes are actually defined. How does a host or internet device determine which address is of which class? Since the length of the network ID is variable (dependent on the class), a simple method was devised to allow the software to determine the class of address and, therefore, the length of the network number.

The IP software will determine the class of the network ID by using a simple method of reading the first bit(s) in the first field (the first byte) of every packet. IP addresses contain 4 bytes. The slide shows an address in binary. If you are not familiar with binary, I suggest you study up on it, for understanding addressing, especially classless addressing, can only be figured out by converting the address to binary.

The slide breaks the IP address down into its binary equivalent. If the first bit of the first byte is a 0, it is a Class A address. If the first bit is a 1, then the protocol mandates reading the next bit. If the next bit is a 0, then it is a Class B address. If the first and second bits are 1 and the third bit is a 0, it is a Class C address. If the first, second, and third bits are 1, the address is a Class D address and is reserved for multicast addresses. Class E addresses are reserved for experimental use.

Identifying a Class