Computer Networks Essentials / Network Layer

Classes of IP Addressing

🤔 What Do We Mean by “Classes of IP Addressing”?

IPv4 addresses are divided into different classes so that:

  • Small networks don’t waste addresses

  • Large networks get enough addresses

  • Routing becomes easier

📌 This method is called Classful Addressing.

🧠 Simple Definition

📌 Classes of IP addressing divide IPv4 addresses into categories (A, B, C, D, E) based on network size and usage.

🧭 Why Were IP Classes Introduced?

Earlier:

  • One address = one network ❌

  • Huge wastage of IP addresses ❌

So IP classes were created to:

  • Allocate addresses based on network size

  • Improve address management

  • Simplify routing

🔢 How IP Classes Are Identified?

📌 IP class is determined by:

  • First octet (first number) of the IP address

Example:

  • 10.x.x.x → Class A

  • 172.16.x.x → Class B

  • 192.168.1.x → Class C

📊 Overview of All IP Classes (Quick Table)

Class

First Octet Range

Network Bits

Host Bits

Usage

A

1 – 126

8

24

Very large networks

B

128 – 191

16

16

Medium networks

C

192 – 223

24

8

Small networks

D

224 – 239

–

–

Multicast

E

240 – 255

–

–

Experimental

📌 127.x.x.x is reserved for loopback (not Class A usage).

🅰️ Class A IP Addressing

🔹 Structure

  • Format: N.H.H.H

  • Network ID: 1 octet

  • Host ID: 3 octets

📌 Example:

10.25.30.40

🧠 Capacity

  • Networks: 126

  • Hosts per network:

    224−2=16,777,2142^{24} - 2 = 16,777,214224−2=16,777,214

📌 Used by:

  • Large ISPs

  • Big organizations

⭐ Key Points

  • Very large number of hosts

  • Huge address wastage

  • Rarely allocated today

🅱️ Class B IP Addressing

🔹 Structure

  • Format: N.N.H.H

  • Network ID: 2 octets

  • Host ID: 2 octets

📌 Example:

172.16.5.10

🧠 Capacity

  • Networks: 16,384

  • Hosts per network:

    216−2=65,5342^{16} - 2 = 65,534216−2=65,534

📌 Used by:

  • Universities

  • Medium-sized enterprises

⭐ Key Points

  • Balanced network & host size

  • Less wastage than Class A

🅲 Class C IP Addressing

🔹 Structure

  • Format: N.N.N.H

  • Network ID: 3 octets

  • Host ID: 1 octet

📌 Example:

192.168.1.10

🧠 Capacity

  • Networks: 2,097,152

  • Hosts per network:

    28−2=2542^{8} - 2 = 25428−2=254

📌 Used by:

  • Homes

  • Small offices

  • LANs

⭐ Key Points

  • Most commonly used class

  • Minimal wastage

  • Easy to manage

🅳 Class D IP Addressing (Multicast)

🔹 Range

  • 224.0.0.0 – 239.255.255.255

📌 Used for:

  • Multicast communication

  • One sender → many receivers

🧠 Examples

  • Video conferencing

  • Online streaming

  • Live broadcasts

📌 No network/host division.

🅴 Class E IP Addressing (Experimental)

🔹 Range

  • 240.0.0.0 – 255.255.255.255

📌 Used for:

  • Research

  • Experimental purposes

❌ Not used in public networks.

🚫 Special IP Addresses (Must Remember)

🔁 Loopback Address

  • 127.0.0.1

  • Tests your own system

📢 Broadcast Address

  • All host bits = 1

  • Sends data to all hosts

Example:

  • 192.168.1.255

🌐 Network Address

  • All host bits = 0

  • Identifies the network

Example:

  • 192.168.1.0

🔐 Private IP Address Ranges (Very Important)

Class

Private Range

A

10.0.0.0 – 10.255.255.255

B

172.16.0.0 – 172.31.255.255

C

192.168.0.0 – 192.168.255.255

📌 Used inside LANs

📌 Not routable on Internet

🧠 Memory Trick to Identify Classes Quickly

First Octet

Class

1–126

A

128–191

B

192–223

C

224–239

D

240–255

E

📌 Odd numbers missing?

127 = Loopback ✔️

⚠️ Limitation of Classful Addressing

  • Massive IP wastage ❌

  • Fixed network sizes ❌

  • No flexibility ❌

👉 This led to:

  • Subnetting

  • CIDR (Classless Addressing)

📌 These are coming next 🚀

⭐ One-Line Summary

🔹 IPv4 addresses are divided into five classes to allocate network and host addresses efficiently based on network size.