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Classless Inter Domain Routing (CIDR) is a method to allocate IP addresses and route IP packets.

I understand how it saves IP address space but I dont understand how it helps the routing process , Can someone enlightened me ??

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In the classful times when an organisation needed 4000 IPv4 addresses there were basically two options:

  • Give them a class B address block with 65536 addresses. This wastes many addresses but is only one route in the routing table's
  • Give them 16 class C address blocks with 4096 addresses in total. This doesn't waste addresses but it does use 16 routing table entries.

With CIDR (classless inter domain routing) we got rid of those classes and started using all prefix lengths. So instead of only being able to give them a /16 or a sixteen /24s it became possible to give them a /20: the required number of addresses in one single routing table entry.

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Sanders has captured the RFC4632 in a simple manner. You can however, check this RFC4632 on why CIDR was implemented.

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