What is it?:

• CIDR (Classless Inter-Domain Routing) is a method for assigning IP addresses and IP routing.
• In AWS VPCs (Virtual Private Clouds), CIDR blocks define the IP address range for your network. For example, 10.0.0.0/16.

Key Concepts:

• IP Range: Specifies the IP range for the VPC using a CIDR block, such as 10.0.0.0/16, where 16 is the subnet mask.
• Private IP Addresses: VPC CIDR blocks often use private IP ranges (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16).
• Subnets: VPC CIDR blocks are divided into subnets for organizing different layers of your network (e.g., public and private subnets).

Important Points:

1. VPC CIDR Block Size: Can range from /16 (65,536 IP addresses) to /28 (16 IP addresses).
2. Multiple CIDR Blocks: A VPC can have multiple CIDR blocks (IPv4 and IPv6) to extend the IP range.
3. Non-overlapping CIDRs: CIDR blocks within a VPC or across peered VPCs must not overlap.
4. Subnet Mask: The smaller the number after the slash (e.g., /16), the larger the range of IP addresses.

Best used for:

• Designing a scalable and secure network structure within AWS.
• Defining network boundaries for resources like EC2 instances, databases, and load balancers.