Plan and visualize subnet allocation within a VPC CIDR block.
Last verified: April 2026
Output will appear here...The planner takes your VPC CIDR block and divides it using a greedy bin-packing algorithm. It sorts the requested subnets by size (largest first), then allocates each one at the next available aligned boundary within the parent CIDR. Alignment ensures that each subnet's network address is a multiple of its size, which is a requirement for valid CIDR blocks.
The Subnet Planner helps you divide a VPC CIDR block into multiple subnets with precise control over sizing, availability zone assignment, and naming. Manually slicing IP ranges is error-prone and tedious, especially when you need public, private, and database subnets across multiple AZs. This tool visualizes the allocation, detects overlaps, and shows remaining free space so you can design production-ready network layouts in minutes.
The classic 3-tier subnet pattern (public/private/data) across 3 AZs needs 9 subnets minimum. Use /20 for private, /22 for data, and /24 for public as a starting point.
Always leave at least 2 unused /20 blocks in your VPC for future tiers. Adding a new subnet tier to a fully allocated VPC means migrating the entire network.
Name your subnets with the pattern {env}-{tier}-{az}, like prod-private-us-east-1a. This naming convention makes IAM policies and NACLs much easier to manage at scale.
AWS allows up to 200 subnets per VPC by default. You can request a quota increase if your architecture requires more, though most designs use between 6 and 20 subnets across availability zones.
Typically no. Private subnets usually need more IP addresses because they host application servers, containers, and databases, while public subnets primarily hold load balancers and NAT gateways. A common pattern is /20 for private subnets and /24 for public subnets.
No. Each AWS subnet exists in exactly one availability zone. To achieve high availability, you create matching subnets in multiple AZs and distribute your resources across them.
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