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What is Classful addressing?

Over the years I've heard the term "CIDR" and never gave it much thought. This morning I perused the Wikipedia article on it, and came away from that read with the understanding that CIDR is really just an algorithm for generating IP addresses for (or on) a network.

Is this a fair assessment? Or have I missed the big picture? Are there other major uses for it?

Thanks in advance!

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marked as duplicate by Dave M, David, bwDraco, Ƭᴇcʜιᴇ007, Nifle Feb 7 '12 at 13:03

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

Well that question was about CIDR's predecessor (classfull addressing), and CIDR is only mentioned a few times to explain that classfull addressing has been replaced (by CIDR). Still no word yet as to what CIDR (or "classless") addressing actually is: an IP-generating algorithm, or otherwise. – pnongrata Feb 6 '12 at 14:35
It is basically a way to "save" IPv4 addresses when some early adopters received an entire Class A range. Really a way to slice up addreses in a more useful way when many "owners" of a Class A used only a fraction of the assigned range. Not really an alogorithim – Dave M Feb 6 '12 at 14:44
An algorithm is a way of doing things. – pnongrata Feb 8 '12 at 12:23
up vote 2 down vote accepted

Apart form it's main use in the Internet, another major use for it is segmenting private IP-address ranges large corporate networks

RFC1519 says

Classless Inter-Domain Routing (CIDR):
         an Address Assignment and Aggregation Strategy

1.  Problem, Goal, and Motivation

   As the Internet has evolved and grown over in recent years, it has
   become evident that it is soon to face several serious scaling
   problems. These include:

      1.   Exhaustion of the class B network address space. One
           fundamental cause of this problem is the lack of a network
           class of a size which is appropriate for mid-sized
           organization; class C, with a maximum of 254 host
           addresses, is too small, while class B, which allows up to
           65534 addresses, is too large for most organizations.

      2.   Growth of routing tables in Internet routers beyond the
           ability of current software, hardware, and people to
           effectively manage.

      3.   Eventual exhaustion of the 32-bit IP address space.

   It has become clear that the first two of these problems are likely
   to become critical within the next one to three years.  This memo
   attempts to deal with these problems by proposing a mechanism to slow
   the growth of the routing table and the need for allocating new IP
   network numbers. It does not attempt to solve the third problem,
   which is of a more long-term nature, but instead endeavors to ease
   enough of the short to mid-term difficulties to allow the Internet to
   continue to function efficiently while progress is made on a longer-
   term solution.
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Thanks RGB! That helps put it into better context – pnongrata Feb 6 '12 at 15:22

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