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

Chapter 38
Classful Addressing—The Original Address Scheme

Many, many years ago, RFC 760 introduced IP. The beginnings of the IP addressing scheme were very simple and flat. This RFC didn’t have a concept of classes (not to be confused with classless IP of today); addressing was an 8–bit prefix that allowed as many as 200+ networks and a lot of hosts per network. RFC 791 obsoletes RFC 760 and this RFC included the concept of IP address classes. Back then, it was easy to change addressing schemes for there were but a few hosts on the entire network. RFC 950 introduced us to subnetting and RFC1518 introduced the CIDR (classless) protocol. There have been many enhancements to the original IP addressing scheme, but they continue to operate on the bases of Class and Classless.

Addressing’s purpose was to allow IP to communicate between hosts on a network or on an internet. Classful IP addresses identify both a particular node and a network number where the particular node resides on an internet. IP addresses are 32–bits long, separated into four fields of 1 byte each. This address can be expressed in decimal, octal, hexadecimal, and binary. The most common IP address form is written in decimal and is known as the dotted decimal notation system.

There are two ways that an IP address is assigned; it all depends on your connection. If you have a connection to the Internet, the network portion of the address is assigned through an Internet Service Provider. Yes, there are three addresses assigned for private addressing. But for a connection to the Internet, at least one address must be defined as a public address assigned to you by the ISP.

To identify all hosts on your network with public address, the ISP will only provide the network range (a continuous IP network address segment) that you may work with. It will not assign host numbers nor assign the network numbers to any part of your network. If your network will never have a connection to the Internet, you can assign your own addresses, but it is highly recommended that you follow RFC 1918 for the private assignment. These are Class A, Class B, and Class C address assignments for private use.