In this section, we will be going to discuss the basics of  IP subnetting in computer networking, benefits of subnetting, host-based and network-based subnetting, CIDR, and VLSM with examples.

Subnetting is a very important topic in the field of networking. It is quite interesting because it involves a lot of mathematical calculations. This tutorial tries to explain each and every term that is related to IP subnetting. I hope it probably helps you a lot to clear your doubt on the subject.

## What is Subnetting

Subnetting is a technique of breaking down a single classful IP network into multiple, logical, and smaller sub-networks or subnets. It helps to create smaller networks out of a single large network. Subnetting is done to control the rapid exhaustion of the IP v4 address. There is unnecessary wastage of host IP addresses while using classful addressing. The wasted IP cannot be allotted to the other network. To overcome this problem, the concept of subnetting is evolved.

## Benefits of Subnetting

The major benefit of subnetting is that it controls the rapid exhaustion of IP addresses. Apart from this, it has other benefits such as;

• Breaks up the large network into smaller manageable segments.
• Reduces network congestion.
• Enhance network security.

## Limitations of  classful address explained with examples.

In classful addressing scheme, Class  A, B, and C have following fixed numbers of host per network using their respective default subnet masks. So, it is clear from the table above that

• Class A has 24 bits for the host, thus the number of hosts per network in Class A will be 224 – 2= 1,67,77,214.
• Class B has 16 bits for the host, thus the number of hosts per network in Class B is 216 – 2 =  65,534
• Class C has 8 bit  for the host, then, the   number of hosts per network in Class C will be 28 – 2 = 254

## Concept of subnetting explained with an example

Now, let us understand, how subnetting helps to overcome the limitation of classful addressing and make the host addressing more flexible using subnetting.

Let us take an example to understand the topic more clearly.

### Examples of Subnetting

You have a Class C network address 192.68.1.0/24.

Now, you can break this Classful network address in two ways in order to create new subnetwork. In the first case, how many equal subnets you want to create by breaking the given classful address and the second case is how many hosts you need in a new subnet

Case1: if we want to create 2 equal subnets out of  the classful address 192.168.1.0/24

We know, default subnet mask of the 192.168.1.0 is 255.255.255.0

In binary it is written as :

11111111.11111111.11111111.00000000

In order to create 2 equal subnets out of 192.168.1.0/24, some host bits must be borrowed from the host to the network portion.

The number of subnet bits that have to be borrowed will be given by the formula,

 2 >= Number of required subnets

In our case, the Number of subnets = 2

Therefore, using the above formula,

we have,  2n >= 2, where n denotes the number of subnet bits.

or,  21 >= 2

Hence; when n=1, we can create two subnets.

Then,  the new subnet mask will be

11111111.11111111.11111111.10000000

In decimal, the new subnet mask is written as 255.255.255.128

Now, there are 7 host bits available for each subnet.

Therefore, the numbers of hosts per subnet are given by

2h -2 = 27– 2 = 126, where h= number of host bits

Each block of subnets will have the maximum numbers of IP addresses as 256 – 128 = 128

The IP range for Subnet-0  is 192.168.1.0 to 192.168.1.127

The IP range for  Subnet-1 is 192.168.1.128 to 192.168.1.255

 Subnets Numer of hosts/subnet Block size/ Total IP address IP Range Valid host IP Network address Broadcast address Subnet-0 126 128 192.168.1.0 to 192.168.1.127 192.168.1.1  to 192.168.1.126 192.168.1.0 192.168.1.127 Subnet-1 126 128 192.168.1.128 to 192.168.1.255 192.168.1.129 to 192.168.1.254 192.168.1.128 192.168.1.255

Case 2: if we want to create a subnet having 100 numbers of hosts

Earlier, when classful addressing is used, we have default 8 host bits for Class C.

Thus, the total number of valid host IP available using Class C default mask is

 28 – 2 = 254

Then, the valid IP will be 192.168.1.1 to 192.168.1.254

Thus, if we use classful addressing, we will have 254 host addresses in hand.

But, our requirement is to create a subnetwork which will have only 100 hosts.

Using a classful address, we are wasting 254-100 = 154 IP addresses that cannot be assigned to any other network.

Here, the concept of subnetting will work to save the wastage of unused IP addresses. This is done by keeping only that much host bits required for 100 hosts and the extra host bits are borrowed by the network. The extra host bits borrowed by the network is known as the subnet bits.

So, the number of  host bits required for 100 hosts is given by the formula

 2 – 2 >= Number of required hosts

Therefore, using the above formula,

we have,  2h – 2 >= 100, where h denotes the number of host bits.

or,  27– 2 >= 100

Hence, 7 bits are kept for the host portion, and the rest bits are borrowed by the network.

Then, we will have a new subnet mask,

11111111.11111111.11111111.1000000

In decimal notation, it  is denoted as 255.255.255.128

Block size of  IP addresses is given by —-> 256 – 128 = 128

Therefore, the range of IP addresses for a new subnet  will be from 192.168.1.0 to 192.168.1.127

Then, the valid host range is from 192.168.1.1 to 192.168.1.126

Here, IP addresses from 192.168.1.128  to 192.168.1 255 are still free to assign to another subnetwork.

From the above example, we have come to the conclusion that subnetting can be done in two ways.

• Subnetting based on the network where the number of subnets is taken as a priority.
• Subnetting based on the host where the host is taken as a priority.

There are a few things that you must know while creating subnets. For subnetting of  any given classful address, the probable questions that arise in our mind are:

• How many network bits required to create each subnet?
• How many host bits available for host IP?
• How many subnets are formed?
• What are the numbers of hosts per subnet?
• What is the modified mask for the new subnets?
• What are the network and the broadcast addresses of the new subnets?
• What is the IP block size of the subnet?
• What is the IP range for the subnets?
• What is the valid host IP range for the subnets?

The solution of these is based on how we approach t to create subnets: whether network-based or host-based. We will solve all these questions in the latter part of the tutorials.

Let us discuss the two types of subnetting in more detail.

## Types of subnetting

The subnetting is done by borrowing host bits to the network part. The borrowing of bits is done in two ways.

• Subnetting based on network or FLSM
• Subnetting based on host  or VLSM

Subnetting based on the network (FLSM): This type of subnetting is done when fixed numbers of subnets are required to be created from the single large network. As a result, each new subnets created have the same subnet mask. Hence, this technique of subnetting is also known as the Fixed Length Subnet Mask ( FLSM).

### FLSM explained with an example

If we want to create 4 subnets out of classful address 192.168.1.0/24 ( here, we are taking Class C address because class C address subnetting is easier to understand. During the subnetting of Class C, only the last octet of the IP address gets affected. We will also explain the concept of subnetting in all three classes of IP addresses separately in the forthcoming section).

To create 4 subnets, we have to borrow the following numbers of host bits to network section;

2n >= 4

or,  22 >=4

Therefore, Network bits required to borrow from the host portion = 2

Thus, the modified subnet mask will be :

11111111.11111111.11111111.11000000

In decimal, we can write as:

255.255.255.192

The block size or the total numbers of IP available for each subnet will be:

256-192 = 64

As we know, the total range of IP available for the given IP address  is from 192.168.1.0 to 192.168.1.255

Hence, starting from 192.168.1.0, we can segment the classful address into 4 equal subnets in a group of 64 blocks.

The distribution of IP for FLSM is mentioned in the table below.

 Subnets Subnets mask CIDR Notation Block size IP Range Usable Host Range Network Address Broadcast Address Subnet-0 255.255.255.192 /26 64 192.168.1.0   to 192.168.1.63 192.168.1.1  to 192.168.1.62 192.168.1.0 192.168.1.63 Subnet-1 255.255.255.192 /26 64 192.168.1.64 to 192.168.1.127 192.168.1.65 to 192.168.1.126 192.168.1.64 92.168.1.127 Subnet-2 255.255.255.192 /26 64 192.168.1.128 to 192.168.1.191 192.168.1.129 to 192.168.1.190 92.168.1.128 192.168.1.191 Subnet-3 255.255.255.192 /26 64 192.168.1.192 to 192.168.1.255 192.168.1.193 to 192.168.1.254 192.168.1.192 192.168.1.255

Now, you can see from the table above that 4 subnets are created with an equal distribution of  64 IP blocks. Each subnet can have 64 IP addresses. FLSM is efficient if each subnet has an equal number of hosts i.e. 64 -2  = 62 (deducting network and the broadcast ID from the total IP block). Each subnet will have the same subnet mask 255.255.255.192 or /26  in CIDR notation.

But, there may be a situation where one subnet may have more than 62 hosts and may have less than 62. In such a case, FLSM or subnetting by the network will not be helpful.

There comes the concept of VLSM. VLSM says that we can create a subnet according to the requirements of the host.

Subnetting based on the host (VLSM): This type of subnetting is done when the host is taken as the first requirement. The subnet is created according to the numbers of hosts in a subnetwork. The subnets created will have a different subnet mask for each subnet. Hence, it is called Variable Length Subnet Mask (VLSM). The VLSM has more benefits over the subnetting based on the network or FLSM. It has better control over the wastage of IP than network-based subnetting.

Let us take an example for a better understanding of VLSM.

### VLSM  explained with an example

Your organization has four different departments viz Accounts, HR, Inventory, and Sales. You want to create separate subnets for each department. However, each department has variable hosts connected to the network such as

Account = 100 hosts

HR = 50 hosts

Inventory = 25 hosts

Sales = 10 hosts

In this case, you have the option to create each subnet with different subnet masks according to the host requirement.

For  Accounts, where 100 hosts are connected,

The number of host bits required to create a subnet for 100 hosts is given by;

2h -2 >= 100, where h= number of required host bit to create subnets for 100 hosts

or, 27 >= 100

or, 126 >= 100

Hence, h = 7

Therefore, the new subnet mask for the subnet: Account is:

11111111.11111111.11111111.10000000

In decimal, it is written as 255.255.255.128

Thus, the block size for Accounts are 256 – 128 = 128

Hence, the range of IP for Account is from 192.168.1.0 to 192.168.1.127

Therefore, the valid host range for Account is from 192.168.1.1 to 192.168.1.126

Next, for the HR department, where 50 hosts are connected,

So, the required number of host bits to connect 50 hosts is given by:

2h -2 >= 50

or, 26 – 2 >= 50

or, 62 >= 50

Hence, h = 6

Therefore, the new subnet mask for the HR is:

11111111.11111111.11111111.11000000

In decimal, it is written as 255.255.255.192

Thus, the block size for HR is 256 – 192 = 64

Hence, the range of IP for the HR department is from 192.168.1.128 to 192.168.1.191

Therefore, the valid host range for HR is from 192.168.1.128 to 192.168.1.190

Next, for the Inventory department, where 25 hosts are connected,

So, the required number of host bits to connect 25 hosts is given by:

2h -2 >= 25

or, 25 – 2 >= 25

or, 30 >= 25

Hence, h = 5

Therefore, the new subnet mask for the Inventory Department  is:

11111111.11111111.11111111.11100000

In decimal, it is written as 255.255.255.224

Thus, the block size for Inventory is 256 – 224 = 32

Hence, the range of IP for Inventory is from 192.168.1.192 to 192.168.1.223

Therefore, the valid host range for Inventory is from 192.168.1.193 to 192.168.1.222

Finally,  for the Sales department, where 10  hosts are connected,

So, the required number of host bits to connect 10 hosts is given by:

2h -2 >= 10

or, 24 – 2 >= 10

or, 14 >= 10

Hence, h =4

Therefore, the new subnet mask for the Sales department is:

11111111.11111111.11111111.11110000

In decimal, it is written as 255.255.255.240

Thus, the block size for Sales is 256 – 240 = 16

Hence, the range of IP for Sales is from 192.168.1.224 to 192.168.1.239