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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,
in this section), it's important to keep
in context what the size of the Internet was when this system was developedit
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
- 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
Of course it turned out that some
of the decisions in the original IP addressing scheme were regrettablebut
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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The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
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