Network CIDR Explained: Private and Public IPs in AWS Networking
AWS Networking: In AWS networking, understanding CIDR (Classless Inter-Domain Routing) is critical for designing scalable, efficient, and secure Virtual Private Clouds (VPCs). CIDR allows flexible IP address assignment using variable-length subnet masking (VLSM), supporting both public and private IP ranges in the cloud.
What is CIDR?
CIDR (Classless Inter-Domain Routing) is a method for allocating IP addresses and routing Internet Protocol packets using a compact representation like 10.0.0.0/16.
CIDR Notation Breakdown
Format: IP_address/prefix_length Example: 192.168.1.0/24
IP Address: 192.168.1.0 Prefix Length: /24 means 255.255.255.0 (first 24 bits are network bits)
CIDR and Subnet Masking
/16 = 65,536 IP addresses /24 = 256 IP addresses /28 = 16 IP addresses
AWS uses CIDR blocks when creating:
VPCs Subnets Route Tables Security Configurations
CIDR in AWS Networking
1. VPC CIDR Block
When creating a VPC in AWS, you define a primary IPv4 CIDR block (e.g., 10.0.0.0/16) Optional: Add secondary CIDR blocks for more subnets using VPC IP address expansion.
2. Subnet CIDR Block
Subnets must use a portion of the parent VPC CIDR. Example:
VPC: 10.0.0.0/16 Subnet 1: 10.0.1.0/24 Subnet 2: 10.0.2.0/24
Private vs. Public IPs in AWS
Private IP Addresses
Definition: IPs not routable over the internet; used for internal communication. Common Ranges (RFC 1918):
10.0.0.0 – 10.255.255.255 (10/8) 172.16.0.0 – 172.31.255.255 (172.16/12) 192.168.0.0 – 192.168.255.255 (192.168/16) Used In:
EC2 internal networking Databases Private Subnets
Public IP Addresses
Definition: Routable over the public internet. Assigned to:
EC2 instances in public subnets Load balancers NAT Gateways (for outbound access) Allocated From:
AWS's pool of IP addresses Or by Elastic IPs (static public IPs)
Practical Use Cases
Scenario 1: Public Web Server
VPC CIDR: 10.0.0.0/16 Subnet CIDR: 10.0.1.0/24 (Public Subnet) Assigned:
Private IP: 10.0.1.10 Public IP: 13.234.56.78 Purpose: Accessible from the internet.
Scenario 2: Private Database Server
Subnet CIDR: 10.0.2.0/24 (Private Subnet) Assigned:
Private IP: 10.0.2.20 No Public IP Purpose: Internal access only (e.g., from Bastion Host or App Layer).
Best Practices
1. Plan CIDR blocks carefully to avoid overlaps in hybrid or multi-VPC setups. 2. Use subnet sizes that match your scaling needs. Don't over-allocate. 3. Split subnets across AZs for high availability. 4. Avoid using small subnet masks like /28 for production services unless truly needed.
Conclusion Understanding CIDR and the use of public and private IPs is foundational for any AWS architect or cloud engineer. It allows you to plan network topology, enable secure access, and scale infrastructure seamlessly within AWS.
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