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CIDR IPv4

🔍 What is CIDR?

CIDR (Classless Inter-Domain Routing) is a method for allocating IP addresses and routing IP packets more efficiently than the old class-based system.

It uses a prefix and a slash notation to represent a range of IP addresses.
Example: 192.168.1.0/24

🧠 CIDR Basics

📌 CIDR Notation

CIDR uses the format:

<IP Address>/<Prefix Length>

  • IP Address → Starting address of the range (in IPv4: 4 octets).

  • Prefix Length → Number of bits used for the network part.

Example: 192.168.1.0/24
This means:

  • First 24 bits = network

  • Remaining 8 bits = hosts

  • 2⁸ - 2 = 254 usable IPs

📌 Binary Representation of IPs

Each IPv4 address is 32 bits, divided into 4 octets (bytes).

For example:

192.168.1.0 =
  11000000 . 10101000 . 00000001 . 00000000

Prefix /24 means:

First 24 bits: Network
Last 8 bits: Hosts

🧮 Calculating CIDR Ranges

Let’s go through CIDR calculations step-by-step:

🔢 How Many IPs in a CIDR Block?

Use the formula:

Number of IPs = 2^(32 - CIDR prefix)
CIDR IPs Usable IPs Subnet Mask
/32 1 1 255.255.255.255
/30 4 2 255.255.255.252
/29 8 6 255.255.255.248
/28 16 14 255.255.255.240
/24 256 254 255.255.255.0
/16 65,536 65,534 255.255.0.0
/8 16,777,216 16,777,214 255.0.0.0

⚠️ Subtract 2 IPs from total:

  • One for network address

  • One for broadcast address

🛠 CIDR Examples

✅ Example 1: 192.168.10.0/24

  • IP Range: 192.168.10.0 – 192.168.10.255

  • Network Address: 192.168.10.0

  • Broadcast Address: 192.168.10.255

  • Usable IPs: 192.168.10.1 – 192.168.10.254

  • Total IPs: 256

✅ Example 2: 10.0.0.0/8

  • IP Range: 10.0.0.0 – 10.255.255.255

  • Usable IPs: 10.0.0.1 – 10.255.255.254

  • Total IPs: 16,777,216

🧩 CIDR vs Subnet Mask

CIDR Subnet Mask
/24 255.255.255.0
/16 255.255.0.0
/8 255.0.0.0

Both represent the same thing, but CIDR is simpler for routing and summarization.

⚒️ Subnetting with CIDR

Suppose you have a /24 block and you want to create 4 subnets:

  • /24 = 256 IPs

  • 4 subnets → need 64 IPs each

  • Each subnet will be /26 (2⁶ = 64)

Subnets:

192.168.1.0/26    → 192.168.1.0 – 192.168.1.63
192.168.1.64/26   → 192.168.1.64 – 192.168.1.127
192.168.1.128/26  → 192.168.1.128 – 192.168.1.191
192.168.1.192/26  → 192.168.1.192 – 192.168.1.255

🌐 CIDR in Real World Use

1. AWS VPC

  • Create a VPC with CIDR block: 10.0.0.0/16

  • Subnets inside:

    • 10.0.1.0/24 → public

    • 10.0.2.0/24 → private

2. Kubernetes

  • Pod CIDR: 10.244.0.0/16

  • Each node might get /24 (256 pods per node)

3. Firewalls & Routing

  • Define CIDR blocks for allow/block rules.

  • Example: Allow only 192.168.0.0/16 traffic.

📘 CIDR Cheat Sheet

Prefix Subnet Mask Hosts
/30 255.255.255.252 2
/29 255.255.255.248 6
/28 255.255.255.240 14
/27 255.255.255.224 30
/26 255.255.255.192 62
/25 255.255.255.128 126
/24 255.255.255.0 254
/23 255.255.254.0 510
/22 255.255.252.0 1022

🔧 Tools

📌 Summary

Concept Meaning
CIDR Notation IP_address/Prefix_length
Network Bits First N bits (CIDR prefix)
Host Bits Remaining 32-N bits
Total IPs 2^(32 - prefix)
Usable IPs Total - 2 (network + broadcast)
Use Cases Routing, Subnetting, AWS VPCs, Kubernetes clusters

Would you like a CIDR practice quiz, PDF version, or a hands-on exercise (e.g., VPC subnetting)?