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CIDR (Classless Inter-Domain Routing) — Modern IP Addressing and Network Design

CIDR (Classless Inter-Domain Routing) is the modern method of allocating IP addresses and defining network boundaries. Unlike the old class-based addressing system (Class A, B, and C), CIDR allows network administrators to create networks of almost any size by specifying the number of network bits using prefix notation (such as /24, /16, or /28). Today, CIDR is used everywhere—from enterprise data centres and cloud platforms to Kubernetes clusters and home networks. Every Linux administrator, DevOps engineer, Cloud Architect, Platform Engineer, Site Reliability Engineer (SRE), and Network Engineer should master CIDR.

Learning Path

Networking Mastery → Module 2: IPv4 Addressing → Lesson 7

Difficulty: Intermediate

Reading Time: 90 Minutes

Course Progress

Course: Networking Mastery

Module: IPv4 Addressing

Lesson: 7 of 10

What You'll Learn

After completing this lesson, you'll be able to:

  • Understand what CIDR is
  • Read CIDR notation
  • Understand prefix lengths
  • Calculate network and host bits
  • Determine the number of hosts in a subnet
  • Understand why CIDR replaced classful addressing
  • Apply CIDR in cloud and enterprise networking

Prerequisites

Complete:

Why Learn CIDR?

Suppose your organisation needs:

500 Hosts

Using classful addressing:

Class C:

254 Hosts

❌ Too Small

Class B:

65,534 Hosts

❌ Too Large

Most addresses would be wasted.

CIDR solves this problem by allowing network sizes that closely match actual requirements.

What is CIDR?

CIDR (Classless Inter-Domain Routing) is a method of allocating IP addresses without relying on predefined address classes.

Instead of using:

Class A

Class B

Class C

CIDR uses:

/8

/16

/24

/26

/28

/30

This provides flexible network sizing.

CIDR Notation

Example:

192.168.1.0/24

The number after the slash indicates:

24 Network Bits

Remaining bits:

8 Host Bits

CIDR Format

IP Address

/

Prefix Length

Example:

10.0.0.0/16

Meaning:

16 Network Bits

16 Host Bits

Prefix Length

The prefix length tells us how many bits belong to the network.

Example:

/24

Binary:

11111111

11111111

11111111

00000000

Equivalent subnet mask:

255.255.255.0

CIDR and Subnet Mask

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
/31 255.255.255.254
/32 255.255.255.255

Network Bits vs Host Bits

Example:

192.168.1.0/24

Binary:

11111111

11111111

11111111

00000000

Network:

24 Bits

Host:

8 Bits

Host Calculation

Formula:

2^(Host Bits)

−

2

The subtraction accounts for the network and broadcast addresses.

Example: /24

Host bits:

8

Calculation:

2⁸

−

2

=

254 Hosts

Example: /26

Host bits:

6

Calculation:

2⁶

−

2

=

62 Hosts

Example: /30

Host bits:

2

Calculation:

2²

−

2

=

2 Hosts

A /30 network is commonly used for point-to-point links.

Common CIDR Blocks

CIDR Hosts
/30 2
/29 6
/28 14
/27 30
/26 62
/25 126
/24 254
/23 510
/22 1022
/21 2046
/20 4094
/16 65,534

CIDR Example

Network:

192.168.10.0/26

Network Address:

192.168.10.0

Usable Hosts:

192.168.10.1

↓

192.168.10.62

Broadcast:

192.168.10.63

Another Example

CIDR:

10.0.0.0/20

Network bits:

20

Host bits:

12

Hosts:

2¹²

−

2

=

4094

Why CIDR Replaced Classful Addressing

Classful addressing:

Class C

↓

254 Hosts

Suppose a company needs:

300 Hosts

Class C:

Too Small

Class B:

65,534 Hosts

Too Large

CIDR allows:

/23

↓

510 Hosts

Much more efficient.

Route Summarisation

CIDR also supports Route Summarisation, where multiple networks are represented by a single route.

Example:

Instead of:

192.168.0.0/24

192.168.1.0/24

192.168.2.0/24

192.168.3.0/24

Summarise as:

192.168.0.0/22

Benefits:

  • Smaller routing tables
  • Faster routing decisions
  • Reduced memory usage

CIDR in Cloud Computing

Cloud providers require CIDR blocks when creating virtual networks.

Examples:

AWS VPC:

10.0.0.0/16

Azure VNet:

10.1.0.0/16

Google Cloud VPC:

172.16.0.0/20

Administrators choose the prefix length based on expected network size.

CIDR in Kubernetes

Kubernetes uses CIDR extensively.

Examples:

Pod Network:

10.244.0.0/16

Service Network:

10.96.0.0/12

Each node receives a smaller CIDR block for Pod allocation.

CIDR in Enterprise Networks

Enterprise environments use CIDR for:

  • Virtual Local Area Networks (VLANs)
  • Branch Offices
  • Virtual Private Network (VPN) Networks
  • Server Networks
  • Storage Networks
  • Demilitarised Zones (DMZs)

Example:

Servers

10.10.10.0/24

Users

10.10.20.0/23

DMZ

10.10.30.0/27

CIDR Calculation Shortcut

Prefix Host Bits
/24 8
/25 7
/26 6
/27 5
/28 4
/29 3
/30 2

Remember:

Host Bits

=

32

−

Prefix

Hands-on Lab

Task 1

Determine the subnet mask for:

/24

/26

/28

/30

Task 2

Calculate the number of usable hosts for:

/25

/26

/27

/28

/29

Task 3

Convert the following subnet masks into CIDR notation:

255.255.255.0

255.255.255.192

255.255.255.240

255.255.0.0

Task 4

Determine the network and broadcast addresses for:

192.168.1.0/26

Task 5

Research the CIDR block used in your cloud lab or home network.

Task 6

Create a VPC design for:

  • Web Tier
  • Application Tier
  • Database Tier

Assign appropriate CIDR blocks to each subnet.

Task 7

Use the ip addr command and identify the prefix length assigned to your network interface.

ip addr

Task 8

For each of the following CIDR blocks, calculate:

  • Subnet mask
  • Total addresses
  • Usable host addresses
10.0.0.0/24

172.16.0.0/20

192.168.100.0/28

Linux Commands

Command Purpose
ip addr Display IP address and prefix
ip route Display routing table
hostname -I Display assigned IP addresses
ping Test connectivity

Common Mistakes

❌ Confusing CIDR with subnet masks.

✅ Learn both representations and convert between them.

❌ Forgetting the network and broadcast addresses.

✅ Subtract two usable addresses for most IPv4 subnets.

❌ Memorising host counts without understanding the formula.

✅ Practice using 2^(host bits) - 2.

❌ Assuming larger prefixes mean larger networks.

✅ Larger prefix numbers mean smaller networks.

❌ Ignoring future growth.

✅ Allocate CIDR blocks with room for expansion.

Best Practices

  • Plan address space before deployment.
  • Leave room for future growth.
  • Use route summarisation where appropriate.
  • Document all CIDR allocations.
  • Avoid overlapping CIDR blocks.
  • Choose subnet sizes based on actual requirements.

Interview Questions

Beginner

  1. What is CIDR?
  2. What does /24 mean?
  3. What is the subnet mask for /26?
  4. Why was CIDR introduced?

Intermediate

  1. Calculate the number of hosts in a /27 network.
  2. Explain the relationship between prefix length and subnet mask.
  3. Why does CIDR improve address utilisation?
  4. What is route summarisation?

Architect Level

  1. Design a CIDR plan for a multi-tier cloud application.
  2. Explain how CIDR reduces routing table size.
  3. How would you allocate CIDR blocks for a Kubernetes cluster and a cloud VPC?

Summary

In this lesson, you learned:

  • What CIDR is
  • CIDR notation
  • Prefix lengths
  • Network and host bits
  • Host calculations
  • CIDR-to-subnet mask conversion
  • Route summarisation
  • Cloud and Kubernetes networking with CIDR

CIDR is the foundation of modern IP networking. It provides flexible address allocation, minimises address wastage, enables efficient routing, and supports scalable enterprise and cloud network designs. Mastering CIDR is essential before learning subnetting and advanced IPv4 planning.

Key Takeaways

  • CIDR replaces the old classful addressing system.
  • The prefix length defines the network portion of an IP address.
  • Host bits are calculated as 32 − Prefix Length.
  • Usable hosts are calculated using 2^(Host Bits) − 2 (for traditional IPv4 subnets).
  • CIDR enables efficient address allocation and route summarisation.

What's Next?

Subnetting