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

Chapter 68
Extending the Life of the IPv4 Address Space

The following was taken from RFC 760:

Addresses are fixed length of four octets (32 bits). An address begins with a one-octet network number, followed by a three-octet local address. This three-octet field is called the rest field.

Taken from RFC 791, page 6:

Addresses are fixed length of four octets (32 bits). An address begins with a network number, followed by a local address (called the rest field). There are three formats or classes of internet addresses: In Class A, the high-order bit is 0, the next 7 bits are the network, and the last 24 bits are the local address; in Class B, the high-order 2 bits are 1–0, the next 14 bits are the network, and the last 16 bits are the local address; In Class C, the high-order 3 bits are 1–1–0, the next 21 bits are the network, and the last 8 bits are the local address.

RFC 950 introduced us to subnetting:

While this view has proved simple and powerful (two-level model, assigning a network number per network), a number of organizations have found it inadequate, and have added a third level to the interpretation of Internet addresses. In this view, a given Internet network is divided into a collection of subnets.

RFCs 1517–1520 introduced us to Classless Inter-Domain Routing (CIDR):

It has become clear that the first two of these problems (routing information overload and Class B exhaustion) are likely to become critical in the near term. Classless Inter-Domain Routing (CIDR) attempts to deal with these problems by defining a mechanism with which to slow the growth of routing tables and reduce the need to allocate new IP network numbers.

This section deals primarily with the IPv4 address extensions. Included in this are subnetting (an IP address review, variable-length subnet masks, route aggregation, and CIDR). IPv6 should be included in this as well with the 128-bit address. However, this discussion is held off until after the IPv4 discussion. The CIDR discussion fully reveals the address problem and what was done about it.