Table of Contents

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.****Controls Broadcast traffic.****Reduces network congestion.****Enhance network security.****Ease of administration.**

## 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**2**^{24}– 2= 1,67,77,214.**Class B**has**16 bits**for the host, thus the number of hosts per network in**Class B**is**2**^{16}– 2 = 65,534**Class C**has**8 bit**for the host, then, the number of hosts per network in**Class C**will be**2**^{8}– 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^{<subnet_bits>} >= Number of required subnets |

In our case, the **Number of subnets = 2**

Therefore, using the above formula,

we have, ** 2 ^{n }>= 2**, where

**n**denotes the

**number of subnet bits.**

or, **2 ^{1 }>= 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

**2 ^{h }-2 = 2^{7}– 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

2^{8} – 2 = 254 |

**Then, the valid IP will be 192.168.1.1 to 192.168.1.254
**

**192.168.1.0** is the network address and **192.168.1.255** is the broadcast address.

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 host**s 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^{<host_bits>} – 2 >= Number of required hosts |

Therefore, using the above formula,

we have, ** 2 ^{h }– 2 >= 100**, where

**h**denotes the

**number of host bits.**

or, **2 ^{7}– 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;

**2 ^{n} >= 4**

or, **2 ^{2} >=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 A**ccounts, 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;

**2 ^{h} -2 >= 100**, where

**h= number of required host bit to create subnets for 100 hosts**

or, **2 ^{7} >= 100**

or, **126 >= 100**

Hence, **h = 7**

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

**11111111.11111111.11111111.1****0000000**

In decimal, it is written as **255.255.255.128**

Thus, the block size for A**ccounts** are **256 – 128 = 128**

Hence, the range of IP for A**ccount** is from **192.168.1.0 to 192.168.1.127**

The network address = **192.168.1.0**

Broadcast address = **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:

**2 ^{h} -2 >= 50**

or, **2 ^{6} – 2 >= 50**

or, **62 >= 50**

Hence, **h = 6**

Therefore, the new subnet mask for the **HR** is:

**11111111.11111111.11111111.1****1000000**

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**

The network address = **192.168.1.128**

Broadcast address = **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:

**2 ^{h} -2 >= 25**

or, **2 ^{5} – 2 >= 25**

or, **30 >= 25**

Hence, **h = 5**

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

**11111111.11111111.11111111.1****1100000**

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**

The network address = **192.168.1.192**

Broadcast address = **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:

**2 ^{h} -2 >= 10**

or, **2 ^{4} – 2 >= 10**

or, **14 >= 10**

Hence, **h =4**

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

**11111111.11111111.11111111.1****1110000**

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**

The network address = **192.168.1.224**

Broadcast address = **192.168.1.239**

Therefore, the valid host range for **Sales ** is from **192.168.1.225 to 192.168.1.238**

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

Subnets | Subnet Mask | CIDR Notation | Block Size | IP Range | Usable Host IP Range | Network Address | Broadcast Address |

Accounts | 255.255.255.128 | /25 | 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 |

HR | 255.255.255.192 | /26 | 64 | 192.168.1.128 to 192.168.1.191 | 192.168.1.129 to 192.168.1.190 | 192.168.1.128 | 92.168.1.191 |

Inventory | 255.255.255.224 | /27 | 32 | 192.168.1.192 to 192.168.1.223 | 192.168.1.193 to 192.168.1.222 | 92.168.1.192 | 192.168.1.223 |

Sales | 255.255.255.240 | /28 | 16 | 192.168.1.224 to 192.168.1.239 | 192.168.1.225 to 192.168.1.238 | 192.168.1.224 | 192.168.1.239 |

I hope this tutorial helps you to understand the concept of subnetting. In the next tutorial, we will discuss subnetting in **Class A, B, and C** separately.