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IP Address classes

Posted on 2007-11-16
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Last Modified: 2011-09-20
This has always made me scratch my head. When it comes to IPv4 address classes, why couldn't a class C address start in 10 or 11. I don't get why the starting octet is sliced up and assigned to the classes.

Isn't it enough that a class A uses the last 3 octets for hosts, the class B uses the last 2 and class C uses the last 1?
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Question by:AnimatorOne
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by:Rogiee
ID: 20301990
it's the man trying to bring us down!
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by:darron_chapman
ID: 20302219
The people involved with creating IP addressing came up with this design.  I believe that they came up with this designation for the simple reason of being to identify quickly how many IPs a particular organization has.  Also, it was an easy way to assign IPs, if a company needed a Class C range, then the assigning company would give them an unused range in the Class C range.  

In 1993, a new way of organizing IPs was introduced, CIDR (Classless Inter-Domain Routing).  This was to replace the somewhat archaic and limited way that IPs were arranged and divided.  Go here for more information.  Here is a great resource for IP addressing.  http://en.wikipedia.org/wiki/IPv4
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by:AnimatorOne
ID: 20302384
I don't think I worded the question clearly.

The range for the first octet of a Class A address is 0 - 126, Class B - 128 - 191, Class C 192 - 223.

Why limit the numbering of the first octet when a class A uses the last 3 octets for hosts, the class B uses the last 2 and class C uses the last 1?
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by:AnimatorOne
ID: 20302396
Rogiee, you're probably right!
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by:darron_chapman
ID: 20302424
You don't have to follow that protocol.  You could divide up a Class A range into a class B range or class C range.  For instance, at my work we use 10.10.0.0 (class A range) with a subnet of 255.255.0.0 (class B subnet) giving us a range of 10.10.0.1 through 10.10.255.254.  The classes are just a nominal designation, nothing more.
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by:AnimatorOne
ID: 20302455
But everything on the subject says:

In Class A networks, the high order bit value (the very first binary number) in the first octet is always 0.
Class B networks have a first bit value of 1 and a second bit value of 0 in the first octet.
Class C networks have a first bit value of 1, second bit value of 1 and a third bit value of 0 in the first octet.

etc.
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by:darron_chapman
ID: 20302505
That is describing the predefined ranges set up originally, and yes technically public ranges do fall under that category.  But within any range, smaller classes can be defined, or CIDR can be used to define the range even further.  Is there something specific you're trying to do?
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by:AnimatorOne
ID: 20302538
I understand on a private network you can do pretty much what you want (might have some troubles with a network 127.0.0.0).

What am I trying to do? I'm trying to understand why the first octet would be used to designate classes rather than the subnet mask. It seems like a waste of addresses. Perhaps this was the method before subnet masks were invented???
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darron_chapman earned 500 total points
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Well, this convention was created well before IPs were at a premium, so yes, I agree, it is a waste of addresses, but the people that came up with this idea didn't think they would need all of them.   They didn't foresee a future where there would be so many computers needing IP addresses.  And yes, while you can do pretty much whatever you want with private network addressing, it is highly recommended to stick with the private addresses defined (10.0.0.0, 172.168.0.0, and 192.168.0.0) because those are considered non routable and routers are programmed to drop anything with that IP.  I hope this has helped a little.
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by:AnimatorOne
ID: 20308683
Thanks Darron
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