🌐 VPC CIDR Subnet Planner
Professional AWS VPC subnet calculator that takes VPC CIDR blocks and generates optimized subnet layouts. Supports even distribution across availability zones, custom subnet sizes, and comprehensive network planning with routing tables and IP allocation details.
VPC Subnet Plan:
10.0.0.0/16 → 6 Subnets Across 3 AZs
65,536 IPs → 10,922 IPs per subnet
📊 Network Overview
🏗️ Subnet Layout Example
📈 IP Allocation Summary
🏗️ Terraform Configuration Preview
How to Use This VPC CIDR Subnet Planner
How to Use the VPC CIDR Subnet Planner
📋 Step 1: Enter VPC CIDR Block
Input your VPC CIDR block in standard notation (e.g., 10.0.0.0/16). The tool supports all RFC 1918 private IP ranges and validates CIDR format automatically.
🔧 Step 2: Choose Planning Mode
- Even Distribution: Creates equal-sized subnets (recommended for most use cases)
- Custom Sizes: Specify individual subnet sizes for complex architectures
- AWS Templates: Use pre-configured patterns for common scenarios
🌐 Step 3: Configure Availability Zones
Select how to distribute subnets across AWS availability zones. Multi-AZ distribution is recommended for production workloads to ensure high availability and fault tolerance.
⚡ Step 4: Set Additional Options
Enable subnet type classification, detailed IP ranges, Terraform code generation, and choose your target AWS region for accurate zone naming.
📊 Step 5: Generate and Analyze
Click "Calculate Subnets" to generate your complete VPC network plan with detailed IP allocations, routing implications, and infrastructure code.
💡 Best Practices
- Reserve at least 25% of your CIDR space for future growth
- Use consistent subnet sizing within each tier (public, private, database)
- Plan for at least 2-3 availability zones for production workloads
- Document your subnet allocations for team reference
How It Works
How the VPC CIDR Subnet Calculator Works
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CIDR Block Validation:
Parses and validates the input VPC CIDR block, checking for proper format, valid IP ranges, and appropriate subnet mask. Ensures the block follows RFC 1918 private address standards.
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IP Address Space Analysis:
Calculates total available IP addresses, determines optimal subnet sizes based on your requirements, and accounts for AWS reserved addresses (network, router, DNS, future use, broadcast).
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Subnet Size Calculation:
For even distribution mode, divides the VPC CIDR space evenly across the requested number of subnets. For custom mode, validates that specified subnet sizes fit within the available address space.
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Availability Zone Distribution:
Maps subnets across selected availability zones using round-robin distribution to ensure balanced network topology and high availability architecture patterns.
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Network Range Generation:
Generates precise CIDR blocks for each subnet, calculates first and last usable IP addresses, and ensures no overlapping ranges or conflicts between subnets.
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Classification and Documentation:
Applies subnet type classifications (public, private, database), generates infrastructure code templates, and provides comprehensive network documentation for implementation.
🔬 Technical Implementation
The calculator uses JavaScript bitwise operations for precise CIDR calculations, implements subnet mask arithmetic for network boundary determination, and validates all calculations against AWS VPC limitations and best practices.
When You Might Need This
- • Plan multi-tier AWS architecture with separate public, private, and database subnets
- • Design high-availability VPC across multiple availability zones for production applications
- • Calculate optimal subnet sizes for microservices deployment with container orchestration
- • Create development environment VPC with isolated testing and staging subnet allocations
- • Plan hybrid cloud network integration with on-premises CIDR block compatibility
- • Design disaster recovery VPC with matching subnet structure across regions
- • Calculate subnet capacity for auto-scaling groups and elastic workloads
- • Plan multi-tenant SaaS architecture with customer isolation via subnet segmentation
- • Design VPC for machine learning workloads with GPU instance subnet optimization
- • Create cost-optimized VPC layout minimizing cross-AZ data transfer charges
Frequently Asked Questions
What's the difference between /16, /20, and /24 CIDR blocks for VPCs?
CIDR notation indicates the number of network bits. /16 gives you 65,536 IP addresses (10.0.0.0/16), /20 provides 4,096 IPs (10.0.0.0/20), and /24 offers 256 IPs (10.0.0.0/24). AWS recommends /16 for large VPCs and /20-/24 for smaller environments. Remember that AWS reserves 5 IPs per subnet for network, router, DNS, future use, and broadcast addresses.
How do I plan subnets across multiple availability zones?
Best practice is to create at least one subnet per availability zone for high availability. For a typical 3-AZ setup, you might create 6 subnets: public and private subnets in each AZ. This allows load balancers to distribute across zones and enables database multi-AZ deployments. The tool automatically distributes subnets evenly across your selected availability zones.
What's the maximum number of subnets I can create in a VPC?
AWS allows up to 200 subnets per VPC by default (can be increased via support). However, practical limitations depend on your CIDR block size. A /16 VPC can theoretically support many /24 subnets, but consider management complexity. Most production VPCs use 6-12 subnets across 2-3 availability zones with public, private, and database tiers.
Can I modify subnet CIDR blocks after creation?
No, subnet CIDR blocks cannot be modified after creation in AWS. You must plan your subnet layout carefully before deployment. However, you can add additional subnets to unused CIDR space within your VPC. This tool helps you plan the complete layout upfront to avoid future constraints and ensure optimal IP address utilization.
How do I avoid CIDR block conflicts with existing networks?
Check existing VPC CIDR blocks, on-premises networks, and VPN connections before planning. AWS recommends using RFC 1918 private ranges: 10.0.0.0/8, 172.16.0.0/12, or 192.168.0.0/16. Avoid overlapping with corporate networks (often 10.x.x.x or 192.168.x.x). The tool validates CIDR format and can help you choose non-conflicting ranges for complex network topologies.