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CIDR Full Form

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  • Difficulty Level : Easy
  • Last Updated : 14 May, 2020
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CIDR stands for Classless Inter-Domain Routing. It is an IP address assigning method that improves the efficiency of address distribution. It is also known as supernetting that replaces the older system based on classes A, B, and C networks. By using a single CIDR IP address many unique IP addresses can be designated. CIDR IP address is the same as the normal IP address except that it ends with a slash followed by a number. It is called IP network prefix.


Characteristics of CIDR

It dynamically allocates the IP addresses by using CIDR blocks on the requirement of the user based on certain rules. The assignment of the CIDR block is handled by the Internet Assigned Number Authority (IANA). CIDR block consists of IP addresses and it consists of some rules:

  • All IP addresses which are allocated to host must be continuous.
  • The block size must be of power 2 and equal to the total number of IP addresses.
  • The size of the block must be divisible by the first IP address of the block.


  • CIDR provide efficient address space allocation i.e. with CIDR addresses are allocated in sizes of any binary multiple.
  • It eliminates the class imbalance, i.e. there are no more class networks so you can widely use some portion of the address space and neglect others.
  • It allows efficient routing entries i.e. a Large number of networks can be represented by a small number of routing entries.
  • No separate subnetting method is required, It implements the concept of subnetting within the internet itself.
  • It aggregates the network description and represents by a single entry.


The main benefit of the older system based on classes was its simplicity. You can determine how many bits of an IP address represents the network ID and how many the host ID by just looking at the first octet. But it is no longer possible, the main drawback of CIDR is its complexity.

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