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🌐 CIDR Notation – Complete Explanation

🌐 CIDR Notation – Complete Explanation

1️⃣ What is CIDR?

CIDR stands for:

Classless Inter-Domain Routing

CIDR is a method used to:

Represent IP address ranges
Perform subnetting
Control network size
Optimize IP usage

2️⃣ Why CIDR Was Introduced (The Real Problem)

Before CIDR, networking used classful addressing.

🔴 Classful IP Problems

Class	Network Size
Class A	Very large
Class B	Medium
Class C	Very small

Problems: ❌ Wasted IP addresses ❌ Inflexible network sizes ❌ Poor internet scalability

✅ CIDR Solved This

CIDR:

Removed fixed classes
Allowed custom network sizes
Improved IP utilization
Enabled modern subnetting

3️⃣ Basic CIDR Format

CIDR notation looks like:

IP_address/prefix_length

Example:

192.168.1.0/24

4️⃣ What Does `/24` Mean?

The /24 is called the prefix length.

It means:

The first 24 bits are network bits

Since IPv4 has 32 bits total:

Network bits = 24
Host bits = 32 - 24 = 8

5️⃣ Understanding IP Addresses in Binary (Very Important)

IPv4 address:

192.168.1.0

In binary:

11000000.10101000.00000001.00000000

Each octet = 8 bits

6️⃣ Network Bits vs Host Bits

CIDR divides IP into:

Network part → identifies network
Host part → identifies devices

Example /24:

Network: 192.168.1
Host:    last octet

7️⃣ How Many IPs Does a CIDR Block Have?

Formula:

Total IPs = 2^(host bits)

Usable IPs:

2^(host bits) - 2

(One for network address, one for broadcast)

8️⃣ CIDR Examples (VERY IMPORTANT)

🔹 Example 1: `/24`

192.168.1.0/24

Host bits = 8
Total IPs = 256
Usable IPs = 254

Range:

192.168.1.0  → Network
192.168.1.1  → First host
192.168.1.254 → Last host
192.168.1.255 → Broadcast

🔹 Example 2: `/16`

10.0.0.0/16

Host bits = 16
Total IPs = 65,536
Usable IPs = 65,534

🔹 Example 3: `/28`

192.168.1.0/28

Host bits = 4
Total IPs = 16
Usable IPs = 14

9️⃣ Common CIDR Blocks You Must Remember

CIDR	Total IPs	Usable
/16	65,536	65,534
/24	256	254
/25	128	126
/26	64	62
/27	32	30
/28	16	14

🔟 CIDR and Subnetting (Connection)

Subnetting is:

Splitting a CIDR block into smaller CIDR blocks

Example:

10.0.0.0/24

Split into two subnets:

10.0.0.0/25
10.0.0.128/25

1️⃣1️⃣ CIDR in AWS (VERY IMPORTANT)

VPC CIDR

When creating a VPC:

10.0.0.0/16

This defines:

Size of your VPC
How many subnets you can create

Subnet CIDR

Subnets use smaller CIDR blocks inside VPC CIDR.

Example:

VPC:     10.0.0.0/16
Subnet1: 10.0.1.0/24
Subnet2: 10.0.2.0/24

1️⃣2️⃣ AWS Reserved IPs (IMPORTANT EXAM POINT)

In AWS:

5 IPs are reserved per subnet

For /24:

Total IPs = 256
Usable = 251

Reserved:

Network address
Router
DNS
Future use
Broadcast

1️⃣3️⃣ CIDR and Route Tables

Route tables use CIDR blocks to:

Match destination IPs
Decide where traffic goes

Example:

0.0.0.0/0 → IGW
10.0.0.0/16 → local

1️⃣4️⃣ CIDR and Security (SG / NACL)

CIDR is used to define:

Allowed IP ranges
Blocked IP ranges

Example:

Allow SSH from 203.0.113.0/24

1️⃣5️⃣ `0.0.0.0/0` (VERY IMPORTANT)

This means:

All IPv4 addresses

Used as:

Default route
Open access

1️⃣6️⃣ Private IP CIDR Ranges (MUST MEMORIZE)

Range	Purpose
10.0.0.0/8	Private
172.16.0.0/12	Private
192.168.0.0/16	Private

1️⃣7️⃣ Longest Prefix Match (Exam Concept)

If multiple routes match:

Route with largest prefix length wins

Example:

10.0.1.0/24 → local
10.0.0.0/16 → IGW

Traffic to 10.0.1.5:

Uses /24 route

1️⃣8️⃣ Real-World Analogy

CIDR is like:

Apartment building size
/16 → big building
/24 → floor
/28 → few flats

1️⃣9️⃣ One-Line Exam Definition

CIDR notation defines an IP address range using a prefix length that determines the network and host portions.

2️⃣0️⃣ Final Summary (Must Remember)

CIDR replaced classful addressing
/number = network bits
Remaining bits = hosts
Used everywhere in AWS
Essential for subnetting, routing, and security