When creating an AWS Virtual Private Cloud (VPC), selecting the appropriate CIDR (Classless Inter-Domain Routing) block is crucial for network design. The CIDR determines your private IP address range and directly impacts:

• Number of available IP addresses
• Subnet segmentation possibilities
• Future expansion capabilities
• Peering and VPN connection options

AWS supports CIDR blocks between /16 (65,536 IPs) and /28 (16 IPs). For most production environments, we recommend:

// Good practice examples
10.0.0.0/16      // Default recommendation
172.16.0.0/20    // Medium-sized deployment
192.168.0.0/24   // Small test environment

Growth Planning: Always account for future expansion. A /16 block provides 256 /24 subnets, while a /20 only gives 16.

Region Strategy: If using multiple regions, coordinate CIDR blocks to avoid conflicts when establishing VPC peering.

While CIDR size doesn't directly affect throughput, poor planning can lead to:

// Problem scenario: Running out of IPs in a test environment
aws ec2 create-vpc --cidr-block 192.168.0.0/28
// Results in only 16 IPs (11 usable after AWS reservations)

Performance impacts occur when you need to re-architect due to insufficient addresses.

For enterprise deployments, consider these Terraform patterns:

# variables.tf
variable "vpc_cidr" {
  description = "CIDR block for the VPC"
  default     = "10.0.0.0/16"
}

# main.tf
resource "aws_vpc" "main" {
  cidr_block           = var.vpc_cidr
  enable_dns_support   = true
  enable_dns_hostnames = true
  tags = {
    Name = "production-vpc"
  }
}

• Overlapping CIDR blocks when peering VPCs
• Using public IP ranges (violates RFC 1918)
• Not reserving enough space for AZ-specific subnets

For complex environments, implement hierarchical addressing:

// Regional allocation pattern
10.{region}.0.0/16  // Where region is numeric (1=us-east-1)
// AZ subnet allocation
10.1.{az}.0/24      // Where az represents availability zone

When creating an AWS VPC, CIDR (Classless Inter-Domain Routing) block selection is crucial for network architecture. The CIDR defines the IP address range for your VPC and affects:

• Number of available IP addresses
• Subnet division possibilities
• Future expansion capability
• Peering and connectivity options

AWS supports CIDR blocks between /16 (65,536 IPs) and /28 (16 IPs). Common recommendations:

# Common production VPC CIDR examples
10.0.0.0/16 (recommended for most use cases)
172.16.0.0/16 (alternative for larger organizations)
192.168.0.0/16 (smaller deployments)

Consider these technical aspects when choosing your VPC CIDR:

1. Current and Future Scaling Needs: Reserve space for growth
2. Subnet Requirements: Multiple Availability Zones need proper addressing
3. Peering/Transit Gateway Compatibility: Avoid overlapping ranges
4. On-Premises Integration: Coordinate with existing corporate networks

While CIDR size doesn't directly impact network performance, it affects:

• Route table complexity (larger CIDRs mean more potential routes)
• Security group management overhead
• NAT gateway efficiency for larger address spaces

Here's how to create VPCs with different CIDR blocks using AWS CLI:

# Create VPC with /16 CIDR (recommended for production)
aws ec2 create-vpc --cidr-block 10.0.0.0/16

# Create VPC with /24 CIDR (development/testing)
aws ec2 create-vpc --cidr-block 10.0.1.0/24

# Create VPC with non-RFC1918 space (requires justification to AWS)
aws ec2 create-vpc --cidr-block 198.51.100.0/24

Watch for these CIDR-related problems:

• IP Exhaustion: When you've allocated all addresses
• Overlap Conflicts: With peered VPCs or on-prem networks
• Subnet Size Miscalculations: AWS reserves 5 IPs per subnet

For complex environments:

# Terraform example with multiple CIDR associations
resource "aws_vpc" "main" {
  cidr_block = "10.0.0.0/16"
  
  # Secondary CIDR for expansion
  assign_generated_ipv6_cidr_block = true
}

# CloudFormation template snippet
"VPC": {
  "Type": "AWS::EC2::VPC",
  "Properties": {
    "CidrBlock": "10.0.0.0/16",
    "EnableDnsSupport": "true",
    "EnableDnsHostnames": "true"
  }
}