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AWS Solutions Architect Associate SAA-C03 Study Notes/
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Amazon VPC/
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Understanding CIDR – IPv4
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Understanding CIDR – IPv4

CIDR (Classless Inter-Domain Routing) is a method for allocating IP addresses and routing IP packets. It is used extensively in AWS networking, including VPC configuration, subnetting, and Security Group rules.

Key Concepts

  • CIDR Notation defines a range of IP addresses using:
    • A Base IP – starting point of the range (e.g., 10.0.0.0)
    • A Subnet Mask – indicates how many bits are fixed for the network portion.
  • Common Examples:
    • WW.XX.YY.ZZ/32 → A single IP address
    • 0.0.0.0/0 → All IP addresses (anywhere)
    • 192.168.0.0/26 → Range from 192.168.0.0 to 192.168.0.63 (64 total IPs)

Subnet Mask Representation

The subnet mask can be expressed in:
  • Slash notation (CIDR): /0, /24, /32, etc.
  • Dotted decimal:
    • /8 → 255.0.0.0
    • /16 → 255.255.0.0
    • /24 → 255.255.255.0
    • /32 → 255.255.255.255

IP Range Examples

CIDR Block
Total IPs
Range Example
192.168.0.0/32
1
192.168.0.0
192.168.0.0/31
2
192.168.0.0 – 192.168.0.1
192.168.0.0/30
4
192.168.0.0 – 192.168.0.3
192.168.0.0/28
16
192.168.0.0 – 192.168.0.15
192.168.0.0/26
64
192.168.0.0 – 192.168.0.63
192.168.0.0/24
256
192.168.0.0 – 192.168.0.255
192.168.0.0/16
65,536
192.168.0.0 – 192.168.255.255
0.0.0.0/0
4,294,967,296
All IPv4 addresses

Quick Memory Guide

  • /32 → No octets change (single IP)
  • /24 → Last octet can change
  • /16 → Last two octets can change
  • /8 → Last three octets can change
  • /0 → All octets can change

AWS Relevance

  • VPC CIDR Block – Defines the overall IP space for your VPC.
  • Subnet CIDR Blocks – Must fit inside the VPC CIDR and cannot overlap.
  • Security Groups & NACLs – CIDR ranges define allowed/denied IPs.
  • Best Practice – Plan CIDR blocks in advance to avoid overlap when peering VPCs.