What Is CIDR Notation in IP Addressing? (With Simple Examples)

If you are learning networking, preparing for CCNA or CCNP, or working as a network engineer, you have probably seen IP addresses written like this:

But what does the “/24” mean?

That slash number is called CIDR notation, and it plays a major role in modern IP networking. In this guide, you will learn:

What CIDR notation is

Why it replaced classful IP addressing

How it works (in simple terms)

How to calculate IP ranges using CIDR

Real examples you can use in exams and real networks

Let’s break it down step by step.

What Is CIDR Notation?

CIDR stands for Classless Inter-Domain Routing.

CIDR notation is a method of representing an IP address along with its network prefix.

The format looks like this:

Example:

The number after the slash (/) tells us how many bits are used for the network portion of the IP address.

Simple Definition (Featured Snippet Style)

CIDR notation is a way to represent an IP address and its network size using a slash (/) followed by the number of network bits.

Why CIDR Was Introduced

Before CIDR, networks used classful IP addressing, which divided IP addresses into fixed classes:

Class A

Class B

Class C

Each class had a fixed subnet mask:

Class A → 255.0.0.0

Class B → 255.255.0.0

Class C → 255.255.255.0

The Problem with Classful Addressing

It caused huge IP address wastage.

Example:
If a company needed 300 IP addresses:

Class C (254 hosts) → Not enough

Class B (65,534 hosts) → Too many

There was no middle option.

CIDR solved this by allowing flexible subnet sizes.

Understanding IP Address Structure First

Before understanding CIDR fully, let’s quickly revise IPv4 basics.

An IPv4 address:

Is 32 bits long

Divided into 4 octets

Each octet = 8 bits

Example:

In binary, it looks like this:

An IP address has two parts:

Network portion

Host portion

CIDR tells us exactly how many bits belong to the network.

How CIDR Notation Works

Let’s understand using a simple example:

Example: 192.168.1.0/24

The “/24” means:

24 bits are for the network

Remaining 8 bits are for hosts

Since IPv4 has 32 bits total:

Subnet Mask Equivalent

/24 means:

Because:

24 network bits = 11111111.11111111.11111111.00000000

How to Calculate Number of IP Addresses

Formula:

Usable Hosts:

(We subtract 2 for network address and broadcast address.)

Simple CIDR Examples (Step-by-Step)

Example 1: 192.168.1.0/24

Host bits = 8

Total IP addresses = 2⁸ = 256

Usable hosts = 256 – 2 = 254

Range:

Network address: 192.168.1.0

First usable: 192.168.1.1

Last usable: 192.168.1.254

Broadcast: 192.168.1.255

This is commonly used in small office networks.

Example 2: 10.0.0.0/16

Host bits:

Total IPs:

Usable:

Subnet mask:

This is often used in enterprise or large internal networks.

Example 3: 192.168.1.0/30

Host bits:

Total IPs:

Usable:

Range:

Network

2 usable IPs

Broadcast

This is commonly used in point-to-point router links.

CIDR vs Classful IP Addressing

CIDR allows network administrators to allocate IP addresses based on actual need.

This helps conserve IPv4 addresses.

CIDR and Route Summarization

CIDR is very important in routing.

Routers use something called:

Longest Prefix Match

Example:

If a router has:

It chooses the most specific route (/24).

CIDR also allows route aggregation (supernetting).

Example:

Instead of advertising:

We can summarize them as:

This reduces routing table size and improves performance.

Protocols that use CIDR:

OSPF

EIGRP

BGP

IS-IS

CIDR in Cloud Networking

If you use AWS, Azure, or GCP, you will see CIDR everywhere.

Example:

When creating a VPC, you must define:

Subnets might be:

Cloud networking completely depends on CIDR-based addressing.

Common CIDR Mistakes

Thinking /24 means 24 hosts (Wrong)

Forgetting to subtract network & broadcast

Mixing subnet mask and prefix length

Not understanding binary boundaries

Calculation errors in exams

Practice is the key to mastering CIDR.

Conclusion

CIDR notation is a flexible and efficient way to represent IP networks.

It:

Eliminates classful IP limitations

Conserves IP addresses

Enables route summarization

Improves routing efficiency

Powers modern enterprise and cloud networks

Understanding CIDR is essential for every networking student and professional.

If you want to master CIDR, start practicing:

/24

/25

/26

/27

/28

/29

/30

The more you calculate, the easier it becomes.

Frequently Asked Questions

What does /24 mean in CIDR notation?

It means 24 bits are used for the network portion and 8 bits for hosts.

How many IP addresses are in /30?

4 total IP addresses, 2 usable.

Is CIDR used in IPv6?

Yes. IPv6 also uses prefix notation like:

How do you convert subnet mask to CIDR?

Count the number of 1s in the subnet mask.

Example:

Why did CIDR replace classful addressing?

Because classful addressing wasted IP addresses and was inefficient.

Why CIDR Notation Is Important for Networking Students

If you are preparing for:

CCNA

CCNP

Network interviews

Cloud certifications

CIDR is a fundamental concept.

Without CIDR knowledge, you cannot:

Perform subnetting

Design networks

Configure routing protocols

Plan IP addressing