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IP Subnet Addressing Overview, Motivation, and Advantages
(Page 1 of 3)
IP addressing was originally designed
around the assumption of a strict two-level
hierarchy for internetworks. The first
level was the network, and the second level the host.
Each organization was usually represented by a single network identifier
that indicated a Class
A, B or C block dedicated to them. Within
that network they had to put all of the devices they wanted to connect
to the public IP network.
The Motivation for Subnet Addressing
It did not take long after the classful
scheme was developed for serious inadequacies in it to be noticed, especially
by larger organizations. The reason is that while dividing a large internetwork
into networks that contain hosts is conceptually simple, it doesn't
always match well the structure of each of the networks that comprises
the internet. A big company with thousands of computers doesn't structure
them as one big whomping physical network. Trying to assign and administer
IP addresses to an organization's entire network without any form of
internal logical structure is very difficult.
Unfortunately, under the original
classful addressing scheme, there was no good solution to
this problem. The most commonly-chosen alternative at the time was to
trade a single large block of addresses such as a Class B for a bunch
of Class Cs. However, this caused additional problems:
- It contributed to the explosion in size of IP
- Every time more address space was needed, the
administrator would have to apply for a new block of addresses.
- Any changes to the internal structure of a company's
network would potentially affect devices and sites outside the organization.
- Keeping track of all those different Class C
networks would be a bit of a headache in its own right.
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The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
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