Subnetting Lesson (Easy Step-by-Step Method)

Lesson Title

Introduction to IPv4 Subnetting

Learning Outcomes

By the end of this lesson, students will be able to:

1. Explain what subnetting is.
2. Identify Network ID and Host ID.
3. Calculate subnet masks.
4. Determine the number of subnets.
5. Determine the number of hosts per subnet.
6. Find the network address, first host, last host, and broadcast address.

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1. What is Subnetting?

Subnetting is the process of dividing one large network into smaller networks (subnets).

Why do we use subnetting?

✅ Reduces network traffic

✅ Improves security

✅ Makes network management easier

✅ Uses IP addresses efficiently

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Real-World Example

Imagine a school with 300 computers.

Instead of one large network:

Department	Subnet
Administration	Subnet 1
Computer Lab	Subnet 2
Library	Subnet 3
Staff Room	Subnet 4

Each department becomes a separate subnet.

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2. IPv4 Address Structure

An IPv4 address contains 32 bits.

Example:

192.168.1.10

Binary form:

11000000.10101000.00000001.00001010

32 bits = 4 octets × 8 bits

192      168      1       10
11000000 10101000 00000001 00001010

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3. Network Portion and Host Portion

Example:

192.168.1.10/24

The "/24" means:

24 bits = Network
8 bits = Host

192.168.1 .10
Network   Host

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4. Understanding CIDR Notation

CIDR	Subnet Mask
/8	255.0.0.0
/16	255.255.0.0
/24	255.255.255.0
/25	255.255.255.128
/26	255.255.255.192
/27	255.255.255.224
/28	255.255.255.240
/29	255.255.255.248
/30	255.255.255.252

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5. Easy 4-Step Subnetting Method

Step 1 – Identify CIDR

Example:

192.168.1.0/26

CIDR = 26

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Step 2 – Find Block Size

Formula:

Block Size = 256 − Last Octet of Subnet Mask

For /26:

Subnet Mask = 255.255.255.192

256 - 192 = 64

Block size = 64

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Step 3 – Create Network Ranges

Start at 0 and count by block size.

0
64
128
192
256

Subnets:

Subnet
192.168.1.0
192.168.1.64
192.168.1.128
192.168.1.192

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Step 4 – Find Host Range and Broadcast

First Subnet

Network:

192.168.1.0

Next subnet:

192.168.1.64

Broadcast:

64 - 1 = 63

Broadcast = 192.168.1.63

Host range:

192.168.1.1
to
192.168.1.62

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Quick Formula

Network Address = First Address

Broadcast Address = Last Address

First Host = Network + 1

Last Host = Broadcast - 1

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6. Host Calculation

Formula:

Hosts = 2^(Host Bits) - 2

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Example 1

Network:

/24

Host bits:

32 - 24 = 8

Calculation:

2^8 - 2

256 - 2

254 Hosts

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Example 2

Network:

/26

Host bits:

32 - 26 = 6

Calculation:

2^6 - 2

64 - 2

62 Hosts

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7. Subnet Calculation

Formula:

Subnets = 2^(Borrowed Bits)

Example:

Default Class C:

/24

New subnet:

/27

Borrowed bits:

27 - 24 = 3

Calculation:

2^3

8 Subnets

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Easy Reference Table

CIDR	Hosts
/24	254
/25	126
/26	62
/27	30
/28	14
/29	6
/30	2

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8. Complete Example

Find subnet information for:

192.168.10.0/27

Step 1

Subnet mask:

255.255.255.224

Step 2

Block size:

256 - 224

32

Step 3

Subnets

0
32
64
96
128
160
192
224

Step 4

First subnet:

Network:

192.168.10.0

Broadcast:

192.168.10.31

Host range:

192.168.10.1
to
192.168.10.30

Hosts:

30

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Classroom Activity 1

Complete the table.

CIDR	Subnet         Mask	Hosts
/25	?	?
/26	?	?
/27	?	?
/28	?	?

Answers

CIDR	Subnet Mask	Hosts
/25	255.255.255.128	126
/26	255.255.255.192	62
/27	255.255.255.224	30
/28	255.255.255.240	14

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Classroom Activity 2

Find:

192.168.5.0/28

1. Network Address
2. Broadcast Address
3. First Host
4. Last Host
5. Number of Hosts

Answer

Network:

192.168.5.0

Broadcast:

192.168.5.15

First Host:

192.168.5.1

Last Host:

192.168.5.14

Hosts:

14

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Memory Trick for Students

1. Find the mask

/27 = 255.255.255.224

2. Find block size

256 - 224 = 32

3. Count by block size

0, 32, 64, 96, 128...

4. Use the range

Network = Start
Broadcast = End
Hosts = Middle

Golden Rule:

Network Address = First IP
Broadcast Address = Last IP
Usable Hosts = Everything in between

This method allows most Class C subnetting questions (/25 to /30) to be solved in less than one minute without converting IP addresses to binary.