## Classless Addressing | CIDR in Networking

There are two systems in which IP Addresses are classified-

1. Classful Addressing System
2. Classless Addressing System

• Classless Addressing is an improved IP Addressing system.
• 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-

When a user asks for specific number of IP Addresses,

• 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-

CIDR IP Addresses look like-

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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