# Classless Addressing | CIDR in Networking

There are two systems in which IP Addresses are classified- • It makes the allocation of IP Addresses more efficient.
• It replaces the older classful addressing system based on classes.
• It is also known as Classless Inter Domain Routing (CIDR).

## CIDR Block-

• CIDR dynamically assigns a block of IP Addresses based on certain rules.
• This block contains the required number of IP Addresses as demanded by the user.
• This block of IP Addresses is called as a CIDR block.

## Rules For Creating CIDR Block-

A CIDR block is created based on the following 3 rules-

## Rule-01:

• All the IP Addresses in the CIDR block must be contiguous.

## Rule-02:

• The size of the block must be presentable as power of 2.
• Size of the block is the total number of IP Addresses contained in the block.
• Size of any CIDR block will always be in the form 21, 22, 23, 24, 25 and so on.

## Rule-03:

• First IP Address of the block must be divisible by the size of the block.

### REMEMBER

If any binary pattern consisting of (m + n) bits is divided by 2n, then-

• Remainder is least significant n bits
• Quotient is most significant m bits

So, any binary pattern is divisible by 2n, if and only if its least significant n bits are 0.

### Examples-

Consider a binary pattern-

01100100.00000001.00000010.01000000

(represented as 100.1.2.64)

• It is divisible by 25 since its least significant 5 bits are zero.
• It is divisible by 26 since its least significant 6 bits are zero.
• It is not divisible by 27 since its least significant 7 bits are not zero.

## CIDR Notation-

a.b.c.d / n

• They end with a slash followed by a number called as IP network prefix.
• IP network prefix tells the number of bits used for the identification of network.
• Remaining bits are used for the identification of hosts in the network.

## Example-

An example of CIDR IP Address is-

182.0.1.2 / 28

It suggests-

• 28 bits are used for the identification of network.
• Remaining 4 bits are used for the identification of hosts in the network.

## Problem-01:

Given the CIDR representation 20.10.30.35 / 27. Find the range of IP Addresses in the CIDR block.

## Solution-

Given CIDR representation is 20.10.30.35 / 27.

It suggests-

• 27 bits are used for the identification of network.
• Remaining 5 bits are used for the identification of hosts in the network.

Given CIDR IP Address may be represented as-

00010100.00001010.00011110.00100011 / 27

So,

• First IP Address = 00010100.00001010.00011110.00100000 = 20.10.30.32
• Last IP Address = 00010100.00001010.00011110.00111111 = 20.10.30.63

Thus, Range of IP Addresses = [ 20.10.30.32 , 20.10.30.63]

## Problem-02:

Given the CIDR representation 100.1.2.35 / 20. Find the range of IP Addresses in the CIDR block.

## Solution-

Given CIDR representation is 100.1.2.35 / 20.

It suggests-

• 20 bits are used for the identification of network.
• Remaining 12 bits are used for the identification of hosts in the network.

Given CIDR IP Address may be represented as-

01100100.00000001.00000010.00100011 / 20

So,

• First IP Address = 01100100.00000001.00000000.00000000 = 100.1.0.0
• Last IP Address = 01100100.00000001.00001111.11111111 = 100.1.15.255

Thus, Range of IP Addresses = [ 100.1.0.0 , 100.1.15.255]

## Problem-03:

Consider a block of IP Addresses ranging from 100.1.2.32 to 100.1.2.47.

1. Is it a CIDR block?
2. If yes, give the CIDR representation.

## Solution-

For any given block to be a CIDR block, 3 rules must be satisfied-

## Rule-01:

• According to Rule-01, all the IP Addresses must be contiguous.
• Clearly, all the given IP Addresses are contiguous.
• So, Rule-01 is satisfied.

## Rule-02:

• According to Rule-02, size of the block must be presentable as 2n.
• Number of IP Addresses in the given block = 47 – 32 + 1 = 16.
• Size of the block = 16 which can be represented as 24.
• So, Rule-02 is satisfied.

## Rule-03:

• According to Rule-03, first IP Address must be divisible by size of the block.
• So, 100.1.2.32 must be divisible by 24.
• 100.1.2.32 = 100.1.2.00100000 is divisible by 24 since its 4 least significant bits are zero.
• So, Rule-03 is satisfied.

Since all the rules are satisfied, therefore given block is a CIDR block.

## CIDR Representation-

We have-

• Size of the block = Total number of IP Addresses = 24.
• To have 24 total number of IP Addresses, total 4 bits are required in the Host ID part.
• So, Number of bits present in the Network ID part = 32 – 4 = 28.

Thus,

 CIDR Representation = 100.1.2.32 / 28

## NOTE-

For writing the CIDR representation,

• We can choose to mention any IP Address from the CIDR block.
• The chosen IP Address is followed by a slash and IP network prefix.
• We generally choose to mention the first IP Address.

## Problem-04:

Consider a block of IP Addresses ranging from 150.10.20.64 to 150.10.20.127.

1. Is it a CIDR block?
2. If yes, give the CIDR representation.

## Solution-

For any given block to be a CIDR block, 3 rules must be satisfied-

## Rule-01:

• According to Rule-01, all the IP Addresses must be contiguous.
• Clearly, all the given IP Addresses are contiguous.
• So, Rule-01 is satisfied.

## Rule-02:

• According to Rule-02, size of the block must be presentable as 2n.
• Number of IP Addresses in given block = 127 – 64 + 1 = 64.
• Size of the block = 64 which can be represented as 26.
• So, Rule-02 is satisfied.

## Rule-03:

• According to Rule-03, first IP Address must be divisible by size of the block.
• So, 150.10.20.64 must be divisible by 26.
• 150.10.20.64 = 150.10.20.01000000 is divisible by 26 since its 6 least significant bits are zero.
• So, Rule-03 is satisfied.

Since all the rules are satisfied, therefore given block is a CIDR block.

## CIDR Representation-

We have-

• Size of the block = Total number of IP Addresses = 26.
• To have 26 total number of IP Addresses, 6 bits are required in the Host ID part.
• So, Number of bits in the Network ID part = 32 – 6 = 26.

Thus,

 CIDR Representation = 150.10.20.64 / 26

## Problem-05:

Perform CIDR aggregation on the following IP Addresses-

128.56.24.0/24

128.56.25.0/24

128.56.26.0/24

128.56.27.0/24

## Solution-

All the 4 given entities represent CIDR block in itself.

We have to now perform the aggregation of these 4 blocks.

## Rule-01:

• According to Rule-01, all the IP Addresses must be contiguous.
• Clearly, all the IP Addresses are contiguous.
• So, Rule-01 is satisfied.

## Rule-02:

• According to Rule-02, size of the block must be presentable as 2n.
• Total number of IP Addresses = 28 + 28 + 28 + 28 = 22 x 28 = 210.
• So, Rule-02 is satisfied.

## Rule-03:

• According to Rule-03, first IP Address must be divisible by size of the block.
• So, 128.56.24.0 must be divisible by 210.
• 128.56.24.0 = 128.56.00011000.00000000 is divisible by 210 since its 10 least significant bits are zero.
• So, Rule-03 is satisfied.

Since all the 3 rules are satisfied, so they can be aggregated.

## CIDR Representation-

We have-

• Size of the block = Total number of IP Addresses = 210.
• To have 210 total number of IP Addresses, 10 bits are required in the Host ID part.
• So, Number of bits in the Network ID part = 32 – 10 = 22.

Thus,

 CIDR Representation = 128.56.24.0/22

## Problem-06:

Perform CIDR aggregation on the following IP Addresses-

200.96.86.0/24

200.96.87.0/24

200.96.88.0/24

200.96.89.0/24

## Solution-

All the 4 given entities represent CIDR block in itself.

We have to now perform the aggregation of these 4 blocks.

## Rule-01:

• According to Rule-01, all the IP Addresses must be contiguous.
• Clearly, all the IP Addresses are contiguous.
• So, Rule-01 is satisfied.

## Rule-02:

• According to Rule-02, size of the block must be presentable as 2n.
• Total number of IP Addresses = 28 + 28 + 28 + 28 = 22 x 28 = 210.
• So, Rule-02 is satisfied.

## Rule-03:

• According to Rule-03, first IP Address must be divisible by size of the block.
• So, 200.96.86.0 must be divisible by 210.
• 200.96.86.0 = 200.96.01010110.00000000 is not divisible by 210 since its 10 least significant bits are not zero.
• So, Rule-03 is unsatisfied.

Since all the 3 rules are not satisfied, so they can not be aggregated.

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Summary Article Name
Classless Addressing | CIDR in Networking
Description
Classless Addressing or CIDR in Networking short for Classless Inter Domain Routing is an improved IP Addressing System. Classless Inter Domain Routing assigns CIDR blocks dynamically to the user asking for specific number of IP Addresses.
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Gate Vidyalay
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