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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
                          9  IP "Classful" (Conventional) Addressing

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IP "Classful" Addressing Network and Host Identification and Address Ranges
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IP "Classful" Addressing Overview and Address Classes
(Page 2 of 2)

Rationale for "Classful" Addressing

While the drawbacks of the “classful” system are often discussed today (and that includes myself as well, later in this section), it's important to keep in context what the size of the Internet was when this system was developed—it was tiny, and the 32-bit address space seemed enormous by comparison to even the number of machines its creators envisioned years into the future. It's only fair to also remember the many advantages of the “classful” system developed over 25 years ago:

  • Simplicity and Clarity: There are only a few classes to choose from and it's very simple to understand how the addresses are split up. The distinction between classes is clear and obvious. The divisions between network ID and host ID in classes A, B and C are on octet boundaries, making it easy to tell what the network ID is of any address.

  • Reasonable Flexibility: Three levels of “granularity” match the sizes of large, medium-sized and small organizations reasonably well. The original system provided enough capacity to handle the anticipated growth rate of the Internet at the time.

  • Routing Ease: As we will see shortly, the class of the address is encoded right into the address to make it easy for routers to know what part of any address is the network ID and what part is the host ID. There was no need for “adjunct” information such as a subnet mask.

  • Reserved Addresses: Certain addresses are reserved for special purposes. This includes not just classes D and E but also special reserved address ranges for “private” addressing.

Of course it turned out that some of the decisions in the original IP addressing scheme were regrettable—but that's the benefit of hindsight. I'm sure we'd all like to have back the 268 odd million addresses that were set aside for Class E. While it may seem wasteful now to have reserved a full 1/16th of the address space for “experimental use”, remember that the current size of the Internet was never anticipated even ten years ago, never mind twenty-five. Furthermore, it's good practice to reserve some portion of any scarce resource for future use. (And besides, if we're going to play Monday morning quarterback, the real decision that should be changed in retrospect was the selection of a 32-bit address instead of a 48-bit or 64-bit one!)

Key Concept: .The “classful” IP addressing scheme divides the IP address space into five classes, A through E, of differing sizes. Classes A, B and C are the most important ones, designated for conventional unicast addresses and comprising 7/8ths of the address space. Class D is reserved for IP multicasting, and Class E for experimental use.

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IP "Classful" (Conventional) Addressing
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IP "Classful" Addressing Network and Host Identification and Address Ranges
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Version 3.0 - Version Date: September 20, 2005

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