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Problems With "Classful" IP Addressing
(Page 1 of 3)
The classful addressing
system was the first major attempt to define a method for universal
addressing of a large IP internetwork. The system had some advantages,
as I mentioned in the
overview of the classful scheme,
and given that it was developed decades ago for a network that was limited
in size, it did the job remarkably well for a long time.
Nobody ever expected the Internet
to mushroom to anything close to its current size. As it grew, problems
become apparent with the classful IP addressing mechanismslowly
at first, but then more rapidly as growth became more rapid. I've hinted
at some of theses problems in my explanation of how this type of addressing
works, but to help frame the discussion of newer addressing types, I
think it is useful to look at this in more detail.
Summary of Classful Addressing Issues
There are three main problems with
classful addressing, which are somewhat related to each
other (making them a bit harder to explain). Let's start with a quick
summary of what these issues are:
- Lack of Internal Address Flexibility:
Big organizations are assigned large, monolithic blocks
of addresses that don't match well the structure of their underlying
- Inefficient Use of Address Space:
The existence of only three block sizes (classes A, B and C) leads to
waste of limited IP address space.
- Proliferation of Router Table Entries:
As the Internet grows, more and more entries are required for routers
to handle the routing of IP datagrams, which causes performance problems
for routers. Attempting to reduce inefficient address space allocation
leads to even more router table entries.
Issue #1 results primarily from the
fact that in the classful system, big companies get assigned
a rather large (Class B) or truly enormous (Class A) block of addresses,
all of which is considered by the Internet routers a single network
with one network ID. Now, imagine that you are running a
medium-to-large-sized company with 5,000 computers, and you are assigned
a Class B address for your network. Do you really have 5,000 computers
all hooked into a single network? I sure as heck hope you don't! Yet
you would be forced to try to fit all of these into a single IP network
in the original classful method. There was no way to create
an internal hierarchy of addresses.
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The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
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