What Is CIDR Notation?

CIDR notation is a simple way to describe how IPv4 networks are divided. It is used instead of long subnet masks and makes network configuration easier to read and understand.

You may have already seen CIDR notation written like this:

192.168.1.0/24
10.0.0.0/8

The number after the slash indicates how many bits are used for the network portion of the IP address.

Why CIDR notation exists

In the early days of the internet, IP addresses were divided into fixed classes: Class A, Class B, and Class C. Each class had a predefined subnet mask.

This approach wasted a large number of IP addresses. Many networks were either too large or too small for the fixed class sizes.

CIDR (Classless Inter-Domain Routing) was introduced to remove these limitations. It allows networks to be split into flexible sizes based on actual needs.

How CIDR notation works

An IPv4 address consists of 32 bits. CIDR notation simply tells how many of those bits belong to the network.

For example:

/8 means 8 bits for the network and 24 bits for hosts
/16 means 16 bits for the network and 16 bits for hosts
/24 means 24 bits for the network and 8 bits for hosts

The remaining bits are used to identify individual devices within the network.

CIDR notation and subnet masks

CIDR notation is directly related to subnet masks. Each CIDR prefix corresponds to a specific subnet mask.

Here are some common examples:

/8  → 255.0.0.0
/16 → 255.255.0.0
/24 → 255.255.255.0

While subnet masks show the same information, CIDR notation is shorter and easier to read.

Why CIDR notation is useful

CIDR notation is widely used because it simplifies many networking tasks. It makes configurations clearer and reduces errors.

It allows flexible network sizes
It reduces wasted IP addresses
It makes routing and firewall rules easier to understand
It is commonly used in cloud and enterprise networks

Common CIDR examples

Some CIDR prefixes are used very frequently in practice:

/24 — small networks (256 total addresses)
/16 — medium-sized networks
/8 — very large networks
/32 — a single IP address

These prefixes help quickly understand the size of a network without doing manual calculations.

How to convert a subnet mask to CIDR

To convert a subnet mask to CIDR notation, you count the number of consecutive 1 bits in the mask.

For example, the subnet mask 255.255.255.0 contains 24 network bits, so it corresponds to /24.

You can use the Netmask to CIDR Converter to perform this conversion instantly without manual calculation.

Summary

CIDR notation is a compact and flexible way to describe IPv4 networks. It replaces rigid class-based addressing and makes modern networking simpler and more efficient.

Understanding CIDR helps you read network configurations, design address spaces, and troubleshoot routing issues more easily.