Tag: Classless Vs Classful Addressing

Classless Addressing | CIDR in Networking

Computer Networks

IP Addressing-

 

There are two systems in which IP Addresses are classified-

 

 

  1. Classful Addressing System
  2. Classless Addressing System

 

In this article, we will discuss about Classless Addressing System.

Learn about Classful Addressing System.

 

Classless Addressing-

 

  • 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.

 

PRACTICE PROBLEMS BASED ON CLASSLESS INTER DOMAIN ROUTING-

 

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.

 

To gain better understanding about Classless Addressing,

Watch this Video Lecture

 

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