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Table Of Contents  The TCP/IP Guide
 9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)
      9  TCP/IP Internet Layer (OSI Network Layer) Protocols
           9  Internet Protocol (IP/IPv4, IPng/IPv6) and IP-Related Protocols (IP NAT, IPSec, Mobile IP)
                9  Internet Protocol Version 4 (IP, IPv4)
                     9  IP Addressing

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IP Address Management and Assignment Methods and Authorities
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IP "Classful" Addressing Overview and Address Classes
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IP "Classful" (Conventional) Addressing

The prior section on IP addressing concepts describes the three different ways that IP addresses are assigned in TCP/IP. The original addressing method worked by dividing the IP address space into chunks of different sizes called classes and assigning blocks of addresses to organizations from these classes based on the size and requirements of the organization. In the early 1980s, when the Internet was in its infancy, this conventional system really had no name; today, to contrast it to the newer “classless” addressing scheme, it is usually called the “classful” IP addressing system.

In this section I describe the first scheme used for IP addressing: so-called “classful” IP addressing. I begin with an overview of the concept and general description of the different classes. I discuss the network and host IDs and address ranges associated with the different classes. I discuss the “capacities” of each of the commonly-used classes, meaning how many networks belong to each, and how many hosts each network can contain. I discuss the special meanings assigned to certain IP address patterns, and also the special ranges reserved for private IP addressing, loopback functions, and multicasting. I conclude with a discussion of the problems with this type of addressing, which led to it eventually being abandoned in favor of subnetting, and eventually, classless assignment of the IP address space.

I should note that the “classful” addressing scheme has now been replaced on the Internet by the newer classless addressing system described later in this section. However, I think it is still important to understand how this original system operates, as it forms the foundation upon which the more sophisticated addressing mechanisms were built. Just keep in mind that the class system isn't really used on the Internet any more.

I should also point out that the word “classful” is also sometimes seen as “classfull”. That would be a misspelling, except, well, “classful” is not really an English word at all. That's why I always put the word “classful” in double-quotes. In fact, I must admit that I pretty much hate the word. It sounds like something an elementary school kid made up because he didn't know the opposite of the word “classless”. (The right word is classed, in case anyone cares.)

My wife suggests the word “classy” would sound nicer than “classful”. She's right, but IP addresses don't wear tuxedos.

Of course, nobody ever asked my opinion on this subject. Not even my wife.

Okay, rant over. J

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IP Address Management and Assignment Methods and Authorities
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Version 3.0 - Version Date: September 20, 2005

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