Computer Networks Essentials / Network Layer

CIDR (Classless Inter-Domain Routing)

🤔 What is CIDR? (Basic Idea)

Earlier, IP addressing used fixed classes:

  • Class A

  • Class B

  • Class C

This caused:

  • ❌ Huge IP wastage

  • ❌ Poor flexibility

  • ❌ Large routing tables

👉 CIDR was introduced to solve these problems.

📌 In simple words:

CIDR allows IP addresses to be allocated without fixed classes.

🧠 Simple Definition

📌 CIDR (Classless Inter-Domain Routing) is an IP addressing method that allows flexible allocation of IP addresses using variable-length subnet masks.

🧠 Why “Classless”?

Because:

  • CIDR removes Class A, B, C boundaries

  • Any number of network bits can be used

  • Addressing becomes flexible and efficient

📌 This is why CIDR is called classless addressing.

🕰️ Why CIDR Was Introduced?

CIDR was introduced to:

  • 📉 Reduce IP address wastage

  • 📦 Support subnetting & supernetting

  • 🚀 Reduce routing table size

  • 🌍 Support Internet growth

👉 CIDR is mandatory for modern Internet routing.

🧠 Key Concept of CIDR

🔑 Prefix Length Notation

CIDR uses slash ( / ) notation.

📌 Format:

IP_Address / Prefix_Length

👉 Example:

192.168.1.0 /24

🔢 What Does /24 Mean?

  • /24 → 24 network bits

  • Remaining bits → host bits

📌 IPv4 total bits = 32

Networkbits=24
Hostbits=32 −24 =8

🧠 CIDR vs Subnet Mask

CIDR:

/24

Subnet Mask:

255.255.255.0

📌 Both mean the same thing.

🧩 CIDR and Subnet Mask Mapping

CIDR

Subnet Mask

/8

255.0.0.0

/16

255.255.0.0

/24

255.255.255.0

/26

255.255.255.192

/30

255.255.255.252

🧠 How CIDR Solves IP Wastage

❌ Without CIDR (Classful)

Need 1000 hosts:

  • Class C → only 254 ❌

  • Class B → 65,534 ❌ (huge waste)

✅ With CIDR

Choose exactly what you need:

/22→1022hosts

✔️ No wastage

✔️ Perfect fit

🪜 CIDR Address Calculation (Step-by-Step)

🔹 Example 1: /26

IP: 192.168.1.0/26

  • Network bits = 26

  • Host bits = 6

Hosts:

2⁶ − 2 = 62

Subnet Mask:

255.255.255.192

🔹 Example 2: /30

IP: 10.0.0.0/30

  • Host bits = 2

Hosts:

2² − 2 = 2

📌 Used for point-to-point links.

🧠 CIDR Block Size (Quick Method)

Block size formula:

256 − subnetmask value

Example:

/26 →255.255.255.192
Block size =256 −192 =64

📦 CIDR Address Range Example

For:

192.168.1.0/26

Item

Address

Network

192.168.1.0

First Host

192.168.1.1

Last Host

192.168.1.62

Broadcast

192.168.1.63

🌐 CIDR & Route Aggregation

CIDR allows supernetting (route aggregation).

Example:

192.168.0.0/24
192.168.1.0/24

👉 Advertised as:

192.168.0.0/23

✔️ Smaller routing table

✔️ Faster routing

⚖️ Classful vs CIDR Addressing

Feature

Classful

CIDR

Address Classes

Fixed (A, B, C)

No classes

Flexibility

Low

High

IP Wastage

High

Very Low

Routing Table

Large

Small

Internet Use

❌ No

✅ Yes

🧠 CIDR + Subnetting + Supernetting Relationship

  • Subnetting → Divide network

  • Supernetting → Combine networks

  • CIDR → Enables both

📌 CIDR is the umbrella concept.

🧠 Very Important CIDR Memory Table

Prefix

Hosts

/24

254

/25

126

/26

62

/27

30

/28

14

/29

6

/30

2

⭐ Advantages of CIDR

  • Efficient IP utilization

  • Flexible addressing

  • Smaller routing tables

  • Supports Internet scalability

❌ Limitations of CIDR

  • Requires good planning

  • Complex for beginners

  • Misconfiguration can cause routing issues

🧠 Memory Trick

  • CIDR → Classless

  • Slash ( / ) → Network bits

  • Bigger / number → Fewer hosts

⭐ One-Line Summary

🔹 CIDR is a flexible, classless IP addressing method that efficiently allocates IP addresses and supports modern Internet routing.