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IP "Classful" Addressing Network and Host Identification and Address Ranges
(Page 1 of 3)
The classful IP addressing
scheme divides the total IP address space into five classes, A through
E. One of the benefits of the relatively simple classful
scheme is that information about the classes is encoded directly into
the IP address. This means we can determine in advance which address
ranges belong to each class. It also means the opposite is possible:
we can identify which class is associated with any address by examining
just a few bits of the address.
This latter benefit was one of the
main motivators for the initial creation of the classful
system, as we saw in the previous topic.
"Classful" Addressing Class Determination Algorithm
When TCP/IP was first created computer
technology was still in its infancy, compared to its current state.
needed to be able to quickly make decisions about how to move IP datagrams
around. The IP
address space was split into classes in
a way that looking at only the first few bits of any IP address would
tell the router where to draw the line between the network
ID and host ID, and thus what to do with the datagram.
The number of bits the router needs
to look at may be as few as one or as many as four, depending on what
it finds when it starts looking. The algorithm used corresponds to the
system used to divide the address space; it involves four very basic
steps (see Figure 61):
Figure 61: Class Determination Algorithm for Classful IP Addresses
The simplicity of the classful IP addressing can be seen in the very uncomplicated algorithm used to determine the class of an address.
- If the first bit is a 0,
it's a class A address and we're done. (Half the address space has a
0 for the first bit, so this is why class A takes up half
the address space.) If it's a 1, continue to step two.
- If the second bit is a 0,
it's a class B address and we're done. (Half of the remaining non-class-A
addresses, or one quarter of the total.) If it's a 1, continue
to step three.
- If the third bit is a 0,
it's a class C address and we're done. (Half again of what's left, or
one eighth of the total.) If it's a 1, continue to step
- If the fourth bit is a 0,
it's a class D address. (Half the remainder, or one sixteenth of the
address space.) If it's a 1, it's a class E address. (The
other half, one sixteenth.)
And that's pretty much it.
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Version 3.0 - Version Date: September 20, 2005
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