VPC/subnet planner
Plan subnets inside a VPC or VNet, see usable addresses after provider reservations, catch overlaps, and export Terraform or CloudFormation.
Runs in your browser · nothing is sentCloud networkingNew
Subnets
| Name | Zone / region | CIDR | Addresses | Usable | |
|---|---|---|---|---|---|
| 256 | 251 | ||||
| 256 | 251 |
Summary
Total usable addresses across all subnets: 502
Reserved addresses per subnet: 5 (AWS).
Overlapping subnets
No overlapping subnets.
Remaining free space in the VPC
These are the largest aligned CIDR blocks that fit in the space not yet used by a subnet, useful as a starting point for the next subnet you add.
10.0.2.0/23 10.0.4.0/22 10.0.8.0/21 10.0.16.0/20 10.0.32.0/19 10.0.64.0/18 10.0.128.0/17
Export
How to use
- Enter the CIDR block of your VPC or VNet and pick a cloud provider.
- Add a row per subnet with a name, CIDR and optional zone.
- Check the usable address count, any overlaps, and the remaining free space.
- Copy the Terraform or CloudFormation export once the plan looks right.
Common scenarios
Add one subnet per zone from the same VPC block and confirm none of them overlap before creating them.
Enter your existing subnets and read the remaining free space to find a CIDR block for the next one.
About this tool
Enter your VPC or VNet CIDR block, add named subnets (for example one per availability zone), and choose a cloud provider. The tool checks that every subnet fits inside the VPC, flags subnets that overlap each other, and subtracts the provider-specific reserved addresses (5 for AWS and Azure, 4 for Google Cloud) from each subnet so the "usable" count is accurate, not just the textbook host count. It also shows the remaining free address space as ready-to-use CIDR blocks, and exports the plan as Terraform or CloudFormation resources. Everything runs locally; nothing is sent anywhere.
🔒 Your input stays in your browser and is never sent anywhere.
Frequently asked questions
Why is the usable address count lower than what a normal subnet calculator shows?
A normal calculator subtracts 2 (network and broadcast address). Cloud providers reserve more: AWS and Azure reserve 5 addresses per subnet (network, 3 for internal use, and broadcast), Google Cloud reserves 4 (the first two and the last two). This tool subtracts the correct number for the provider you pick.
What counts as an overlap?
Two subnets overlap if any address range is shared between them, which is invalid in every cloud provider. The tool checks every pair of subnets you have entered and lists which ones conflict.
What is the "remaining free space" list for?
It shows the largest aligned CIDR blocks that fit in whatever part of the VPC is not yet covered by one of your subnets, which is the correct starting point if you want to add another subnet without creating a gap or an invalid block.
Does the Terraform or CloudFormation export include everything in my real setup?
No. It is a minimal starting point (a VPC and a subnet resource per row, with the availability zone if you set one), meant to save typing the CIDR blocks by hand. Add route tables, NAT gateways, tags and other resources as needed for your actual infrastructure.
What to do next
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