IPv4 Subnetting - My Computer Notes

Introduction

As computer networks grow, managing IP addresses efficiently becomes increasingly important. IPv4 subnetting is a fundamental networking technique that helps network administrators divide large networks into smaller, more manageable segments called subnets.

Whether you’re preparing for networking certifications, managing enterprise networks, or simply learning how the Internet works, understanding subnetting is an essential skill.

This guide explains IPv4 subnetting, the formulas involved, and practical examples to help you master the concept.

Why Do We Need Subnetting?

Without subnetting, all devices would exist within a single large network, leading to:

Increased broadcast traffic that can lead to network congestion and unnecessary use of bandwidth.
Reduced performance may occur as a larger network experiences higher levels of traffic and communication overhead.
Poor network management makes it more difficult for administrators to monitor, troubleshoot, and maintain the network effectively.
Security challenges arise because a larger network provides more opportunities for unauthorized access and the spread of security threats.
Inefficient IP address allocation can result in wasted address space and difficulties in managing network resources.

Subnetting solves these issues by dividing a network into smaller logical networks.

Benefits include:

Better network performance is achieved by reducing broadcast traffic and limiting network congestion.
Improved security is provided by separating devices into different subnets, making it easier to control access and contain threats.
Efficient IP utilization allows IP addresses to be allocated according to the specific needs of each subnet, reducing waste.
Easier troubleshooting is possible because network issues can be isolated to a specific subnet, making diagnosis and resolution faster.
Simplified network management helps administrators organize, monitor, and maintain the network more effectively.

Components Required to Calculate Subnetting

An IP address uniquely identifies a device on a network and enables communication between devices.
A subnet mask is used to distinguish the network portion of an IP address from the host portion.
The prefix length indicates the number of bits allocated to the network portion of an IP address and is represented using CIDR notation (e.g., /24).
The number of subnet bits borrowed from the host portion determines how many subnets can be created within a network.
The number of subnets is calculated based on the number of borrowed bits using the formula $2^n$ 2n, where $n$ n is the number of subnet bits.
The number of hosts per subnet is determined by the remaining host bits and is calculated using the formula $2^h – 2$ , where $h$ the number of host bits.

Once the above information is gathered or calculated from the given IP address, we can easily calculate the following:

New subnet mask
Number of subnets
Number of hosts per subnet
Network address
First usable host address
Last usable host address
Broadcast address
Block Size of IP Address Range

Key Subnetting Formulas

Number of Subnets = 2ⁿ, where n is the number of bits borrowed from the host portion.
Number of Hosts per Subnet = 2^h – 2, where h is the number of host bits remaining after borrowing. The first and the last IP addresses are not included in the valid host IP address range. The first one is the network address, and the last one is the broadcast address.
Block Size = 256 – Last Octet of Subnet Mask

These three formulas solve most subnetting problems.

Subnetting is the process of dividing a larger IP network into smaller, more manageable sub-networks (subnets). Here’s a clear example to help you understand it step by step.

Example of Subnetting a Class C Network

Create 4 subnets from the following network.

Let’s start with this network:

IP address: 192.168.1.0
Default subnet mask: 255.255.255.0 (/24)

This means:

Total IP addresses = 256 (from 192.168.1.0 to 192.168.1.255) using default subnet mask

To create 4 subnets:

Use the formula: No of Subnets = ( 2ⁿ ) = ( 2⁴ )
So, 2 host bits to be borrowed by the network portion.
The new network bits : 24 + 2 = 26
The new subnet mask : 11111111.11111111.11111111.11000000 = 255.255.255.192
Now, the remaining host bit : (32 – 26) = 6 host bits
Hence, the no. of hosts per subnets = ( 2⁶ ) = 64

Each subnet will have the IP range as shown below.

Subnets	Network Address	Usable IP Range	Broadcast Address
1	192.168.1.0	192.168.1.1 to 192.168.1.62	192.168.1.63
2	192.168.1.64	192.168.1.65 to 192.168.1.126	192.168.1.127
3	192.168.1.128	192.168.1.129 to 192.168.1.190	192.168.1.191
4	192.168.1.192	192.168.1.193 to 192.168.1.254	192.168.1.25

Variable Length Subnet Masking (VLSM)

VLSM (Variable Length Subnet Mask) is a subnetting technique that allows you to use different subnet masks for different subnets within the same network. This helps use IP addresses more efficiently.

Why VLSM is needed

With traditional subnetting (fixed-length subnet mask), all subnets must be the same size, even if some need only a few hosts and others need many.

VLSM Rules

Start with the largest host requirement.
Assign the appropriate subnet.
Move to the next available address block.
Continue until all requirements are met.

A common formula used when designing VLSM is:

2^h – 2 >= Required Hosts

where h is the number of host bits and -2 indicates the network address and broadcast address

Example:

Department A needs 100 hosts
Department B needs 50 hosts
Department C needs 10 hosts

Using the same subnet mask for all three wastes many IP addresses.

With VLSM:

Department A gets a larger subnet.
Department B gets a medium subnet.
Department C gets a smaller subnet.

Example

Suppose you have the network:

192.168.1.0/24

Requirements: You need three different subnets:

Department A with 100 hosts
Department B with 50 hosts
Department C with 25 hosts

Use the following formula to calculate number of host bits required to

2^h−2 ≥ Required Hosts

The following table contains binary-to-decimal conversions and will be of great help while calculating the required host bits for each subnet.

2⁸	2⁷	2⁶	2⁵	2⁴	2⁷	2²	2¹
256	128	64	32	16	8	4	2

Step 1: Allocate the largest subnet first

For a department A with 100 hosts

2^h−2 ≥ 100

or 2⁷−2 ≥ 100

Using 7 host bits, the maximum host address available will be given by 2⁷ = 128 hosts

Hence, the available network bits will be 32 – 7 = 25

The new subnet for Department A:

The network ID will be 192.168.1.0/25
The new subnet mask will be 11111111.11111111.11111111.10000000 = 255.255.255.128
Block size = 256 – 128 = 128
Host range is from 192.168.1.0 to 192.168.0.127
Network address = 192.168.1.0
Broadcast address = 192.168.1.127
Usable host range:192.168.1.1 – 192.168.1.126

Step 2: Allocate the next largest subnet

For a department B with 50 hosts

2^h−2 ≥ 50

or 2⁶−2 ≥ 50

Using 6 host bits, the maximum host address available will be given by 2⁶ = 64 hosts

Hence, the available network bits will be 32 – 6 = 26

The new subnet for Department B:

The network ID will be 192.168.1.128/26
The new subnet mask will be 11111111.11111111.11111111.11000000 = 255.255.255.192
Block size = 256 – 192 = 64
Host range is from 192.168.1.128 to 192.168.0.191
Network address = 192.168.1.128
Broadcast address = 192.168.1.191
Usable host range:192.168.1.128 – 192.168.1.190

Step 3: Allocate the smallest subnet

For a department C with 25 hosts,

2^h−2 ≥ 50

or 2⁶−2 ≥ 25

Using 5 host bits, the maximum host address available will be given by 2⁵ = 32 hosts

Hence, the available network bits will be 32 – 5 = 27

The new subnet for Department B:

The network ID will be 192.168.1.192/26
The new subnet mask will be 11111111.11111111.11111111.11100000 = 255.255.255.224
Block size = 256 – 224 = 32
Host range is from 192.168.1.192 to 192.168.0.223
Network address = 192.168.1.192
Broadcast address = 192.168.1.223
Usable host range: 192.168.1.193 – 192.168.1.222

	Dept A	Dept B	Dept C
No of Hosts	100	50	25
Host Bit Required	7	6	5
Maximum host Range	128	64	32
IP Address Range	192.168.1.0 to 192.168.1.127	192.168.1.128 to 192.168.1.191	192.168.1.192 to 192.168.1.223
Valid Host Range	192.168.1.1 to 192.168.1.126	192.168.1.129 to 192.168.1.190	192.168.1.193 to 192.168.1.222
Subnet mask	255.255.255.128	255.255.255.192	255.255.255.224

Subnetting a Class B Address

A Class B IP address uses the default subnet mask:

Default Mask: 255.255.0.0
CIDR Notation: /16

Suppose we have:

Network Address: 172.16.0.0/16

Now we want to create 4 subnets out of the given Class B address.

To create 4 subnets:

2 bits be borrowed from the host portion.
Number of subnets = 2ⁿ= 4 , where n=2 is the subnet bits borrowed from the host portion.
Now, the new subnet mask will be 16 +2 = 18 ( network bits)
In binary, written as 11111111 . 11111111 . 11000000 . 00000000
New Subnet mask for all subnets = 255.255.192.0

The subnet ranges are:

1st subnet = 172.16.0.0 to 172.16.63.255
2nd subnet = 172.16.64.0 to 172.16.127.255
2nd subnet = 172.16.128.0 to 172.16.191.255
2nd subnet = 172.16.192.0 to 172.16.63.255

The network address, broadcast address, and the valid host range are

Subnet	Network Address	Host Range	Broadcast Address
1	172.16.0.0/18	172.16.0.1 to 172.16.63.254	172.16.63.255
2	172.16.64.0/18	172.16.64.1 to 172.16.127.254	172.16.127.255
3	172.16.128.0/18	172.16.128.1 to 172.16.191.254	172.16.191.255
4	172.16.192.0/18	172.16.192.1 to 172.16.255.254	172.16.255.255

Quick Reference Table for Class B Address

The following table is of great help while subnetting the Class B address.

CIDR	Subnet Mask	Number of Subnets	Hosts/Subnet
/17	255.255.128.0	2	32766
/18	255.255.192.0	4	16382
/19	255.255.224.0	8	8190
/20	255.255.240.0	16	4094
/21	255.255.248.0	32	2046
/22	255.255.252.0	64	1022
/23	255.255.254.0	128	510
/24	255.255.255.0	256	254

Subnetting in Class A Address

A Class A IP address uses the default subnet mask:

Default Mask: 255.0.0.0
CIDR Notation: /8