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IPv4 Classes — Understanding Classful IP Addressing

In the early days of the Internet, IPv4 addresses were divided into predefined classes known as Class A, B, C, D, and E. Each class was designed to support networks of different sizes, from small organisations to global enterprises. Although modern networks primarily use CIDR (Classless Inter-Domain Routing), understanding IPv4 classes remains important because they are frequently discussed in networking courses, certification exams, and technical interviews. Every Linux administrator, DevOps engineer, Cloud Architect, Platform Engineer, Site Reliability Engineer (SRE), and Network Engineer should understand IPv4 classes and their historical significance.

Learning Path

Networking Mastery → Module 2: IPv4 Addressing → Lesson 3

Difficulty: Beginner

Reading Time: 90 Minutes

Course Progress

Course: Networking Mastery

Module: IPv4 Addressing

Lesson: 3 of 10

What You'll Learn

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

  • Understand IPv4 classful addressing
  • Identify Class A, B, C, D, and E addresses
  • Explain default subnet masks
  • Calculate network and host capacities
  • Understand why CIDR replaced classful networking
  • Recognise IPv4 classes in real-world scenarios

Prerequisites

Complete:

Why Learn IPv4 Classes?

Before CIDR was introduced, routers determined the network size based on the first octet of an IP address.

Example:

10.20.30.40

Immediately tells us:

Class A

Similarly,

172.16.5.10

↓

Class B

192.168.1.50

↓

Class C

Although modern networking uses CIDR, understanding these classes makes subnetting and IP planning much easier.

What is Classful Addressing?

Classful Addressing divides the IPv4 address space into five predefined classes.

Class A

Class B

Class C

Class D

Class E

Each class has:

  • A specific address range
  • A default subnet mask
  • A different number of networks
  • A different number of hosts

IPv4 Class Overview

Class First Octet Default Mask Purpose
A 1–126 255.0.0.0 (/8) Very Large Networks
B 128–191 255.255.0.0 (/16) Medium Networks
C 192–223 255.255.255.0 (/24) Small Networks
D 224–239 N/A Multicast
E 240–255 N/A Experimental

Class A

Range:

1.0.0.0

↓

126.255.255.255

Default subnet mask:

255.0.0.0

(/8)

Network bits:

8

Host bits:

24

Binary Pattern

Class A addresses always begin with:

0

Example:

10.10.20.30

Binary (first octet):

00001010

Leading bit:

0

Capacity

Item Value
Networks 126
Hosts per Network 16,777,214

Formula:

2²⁴ − 2

=

16,777,214

(Two addresses are reserved: network and broadcast.)

Common Example

10.0.0.0/8

Widely used in:

  • Large enterprises
  • Cloud networks
  • Data centres

Class B

Range:

128.0.0.0

↓

191.255.255.255

Default subnet mask:

255.255.0.0

(/16)

Network bits:

16

Host bits:

16

Binary Pattern

Class B addresses begin with:

10

Example:

172.16.10.5

Binary (first octet):

10101100

Leading bits:

10

Capacity

Item Value
Networks 16,384
Hosts per Network 65,534

Formula:

2¹⁶ − 2

=

65,534

Common Example

172.16.0.0/16

Often used by medium-sized organisations.

Class C

Range:

192.0.0.0

↓

223.255.255.255

Default subnet mask:

255.255.255.0

(/24)

Network bits:

24

Host bits:

8

Binary Pattern

Class C addresses begin with:

110

Example:

192.168.1.10

Binary (first octet):

11000000

Leading bits:

110

Capacity

Item Value
Networks 2,097,152
Hosts per Network 254

Formula:

2⁸ − 2

=

254

Common Example

192.168.1.0/24

Used in:

  • Homes
  • Small businesses
  • Labs

Class D

Range:

224.0.0.0

↓

239.255.255.255

Purpose:

Multicast

Used for:

  • Video streaming
  • IPTV
  • Routing protocols
  • Group communication

Class D addresses are not assigned to individual hosts.

Class E

Range:

240.0.0.0

↓

255.255.255.255

Purpose:

Experimental

Research

These addresses are generally not used in normal production networks.

Reserved Addresses

Some IPv4 addresses are reserved.

Address Purpose
0.0.0.0 Default/Unspecified
127.0.0.0/8 Loopback
255.255.255.255 Limited Broadcast

These ranges have special meanings and cannot be assigned as normal host addresses.

Class Comparison

Class Network Bits Host Bits Default Mask
A 8 24 /8
B 16 16 /16
C 24 8 /24

Why Did Classful Addressing Fail?

Suppose a company needed:

500 Hosts

Class C:

254 Hosts

❌ Too Small

Class B:

65,534 Hosts

❌ Too Large

This caused significant IP address wastage.

Introduction of CIDR

To solve address wastage, CIDR (Classless Inter-Domain Routing) was introduced.

Instead of:

Class A

Class B

Class C

CIDR allows:

/22

/26

/19

/30

/27

Networks can now be sized according to actual requirements.

Real-World Examples

IP Address Class
10.5.10.20 A
172.20.15.8 B
192.168.1.100 C
230.10.20.5 D
245.1.2.3 E

Production Perspective

Modern enterprise networks rarely use classful routing.

Instead, they rely on:

  • CIDR
  • Variable Length Subnet Masking (VLSM)
  • Route Summarisation

However, understanding IPv4 classes helps interpret default masks and recognise private address ranges.

Cloud Perspective

Cloud providers use CIDR instead of classes.

Example Virtual Private Clouds (VPCs):

10.0.0.0/16

172.20.0.0/20

192.168.100.0/24

Although these ranges originate from Class A, B, and C private spaces, cloud networking treats them as flexible CIDR blocks.

Kubernetes Perspective

Kubernetes clusters also use CIDR.

Examples:

10.244.0.0/16

10.96.0.0/12

Classful boundaries are ignored in favour of CIDR notation.

Hands-on Lab

Task 1

Identify the class of the following IP addresses:

10.5.20.30

172.16.10.5

192.168.100.10

224.0.0.5

245.10.20.30

Task 2

Write the default subnet mask for:

  • Class A
  • Class B
  • Class C

Task 3

Determine the network and host bits for each class.

Task 4

Display your IPv4 address.

ip addr

Identify which historical class it belongs to.

Task 5

Convert the first octet of these addresses to binary and identify the class using the leading bits.

Task 6

Calculate the maximum number of hosts for:

  • Class A
  • Class B
  • Class C

Task 7

Research why CIDR replaced classful addressing and summarise the advantages.

Task 8

Create a comparison table showing Classes A–E, including:

  • Address Range
  • Default Mask
  • Purpose
  • Host Capacity

Binary Patterns

Class Leading Bits
A 0
B 10
C 110
D 1110
E 1111

Linux Commands

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

Common Mistakes

❌ Thinking classes are used for modern routing.

✅ Modern routing uses CIDR.

❌ Confusing private ranges with classes.

✅ A class defines address structure; private ranges are reserved blocks.

❌ Memorising ranges without understanding default masks.

✅ Learn how masks relate to network and host bits.

❌ Assuming Class D hosts can be assigned to devices.

✅ Class D is reserved for multicast.

❌ Ignoring historical significance.

✅ Many certifications and interviews still reference IPv4 classes.

Best Practices

  • Understand IPv4 classes before learning CIDR.
  • Memorise the first-octet ranges for Classes A, B, and C.
  • Learn the default subnet masks.
  • Understand why CIDR replaced classful networking.
  • Focus on concepts rather than memorisation alone.

Interview Questions

Beginner

  1. What are IPv4 address classes?
  2. What is the default subnet mask for Class C?
  3. Which class is used for multicast?
  4. Which class contains the address 172.16.10.5?

Intermediate

  1. Compare Class A, B, and C addressing.
  2. Why was Classful Addressing inefficient?
  3. Explain the purpose of Classes D and E.
  4. How many hosts can a Class C network support?

Architect Level

  1. Why did the Internet transition from classful addressing to CIDR?
  2. How does CIDR improve address utilisation?
  3. Why is understanding IPv4 classes still valuable in enterprise networking?

Summary

In this lesson, you learned:

  • IPv4 Classes A–E
  • Address ranges
  • Default subnet masks
  • Network and host capacities
  • Binary class identification
  • Limitations of classful addressing
  • Why CIDR replaced classful networking

Although modern networking uses CIDR, IPv4 classes remain an important foundational concept. Understanding how addresses were historically divided helps explain subnet masks, private address ranges, and the evolution of IP addressing.

Key Takeaways

  • IPv4 originally used five address classes.
  • Class A, B, and C were used for host addressing.
  • Class D is reserved for multicast.
  • Class E is reserved for experimental purposes.
  • CIDR replaced classful addressing to improve IP address utilisation.

What's Next?

Private vs Public IP