Introduction
cidrcalculator.net is a free, browser-based toolkit for network engineers and cloud teams. Everything runs client-side — your IP ranges never leave your machine unless you explicitly save a project.
If you're new, start with the calculator, work through a few lessons, then try the VLSM designer.
Quick start
- Open the calculator and enter a CIDR block (e.g.
10.0.0.0/16) - Drag the prefix slider to see how the math changes in real time
- Switch to VLSM to split that network into smaller subnets
- Pick a cloud mode to apply provider-specific reserved IPs
- Export to your IaC tool of choice
Key concepts
CIDR notation
CIDR (Classless Inter-Domain Routing) replaced the old "class A/B/C" system. A CIDR block is written as IP/prefix, where the prefix tells you how many bits are "network" bits.
192.168.1.0/24— first 24 bits are network, last 8 are host (256 addresses)10.0.0.0/8— first 8 bits are network, last 24 are host (16.7M addresses)
Reserved IPs
Every subnet reserves the first address (network) and last address (broadcast). Cloud providers reserve more — see the Cloud Modes page.
Private vs Public ranges
RFC 1918 defines three private ranges that are not routable on the internet: 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16. Use these for internal networks; use public ranges only when you own them.
Calculator
The main calculator supports IPv4 and IPv6, with:
- Live bit-level visualization of network/host bits
- All eight derived values (network, broadcast, mask, wildcard, first/last usable, count, class)
- Binary, hexadecimal and decimal conversions
- Reverse DNS (in-addr.arpa) format
- Overlap detection between any two networks
- Supernet aggregation (combine multiple CIDRs into one)
- Range-to-CIDR (paste a start/end IP, get the smallest covering blocks)
VLSM designer
Variable Length Subnet Masking lets you carve a network into different-sized subnets. The tree designer makes this visual:
- Split — halve a subnet (e.g. /24 → two /25s)
- Join — merge two adjacent subnets back together
- Notes & colors — tag each subnet with purpose, owner, environment
- Templates — start from common patterns (3-tier app, multi-AZ, hub-spoke)
- Share — generate a permalink encoding your design
Cloud modes
Each cloud provider reserves a different number of IPs per subnet. Selecting a cloud mode applies those rules:
- AWS VPC — 5 reserved (first 4 + last 1), minimum /28
- Azure VNet — 5 reserved, minimum /29
- Google Cloud — 4 reserved (first 2 + last 2), minimum /29
- Oracle Cloud — 3 reserved, minimum /30
- Standard — 2 reserved (network + broadcast)
Infrastructure as Code export
Once you've designed your subnets, export to any of 12 IaC formats: Terraform (AWS/Azure/GCP), CloudFormation, Bicep, ARM, Pulumi (TS/Python), AWS CDK, Ansible, Kubernetes NetworkPolicy, Cisco IOS.
Generated code accounts for cloud-specific reserved IPs and includes proper tagging. Always review before applying to production.
Visualization
Five visualization modes:
- Subnet tree — hierarchical view
- VPC topology — auto-generated network diagram
- Bit map — color-coded network/host bits
- IP heatmap — see allocation density across a large range
- Mermaid — code-as-diagram for embedding in docs
Export to SVG, PNG, Mermaid, or draw.io XML.
AI assistant
Describe what you need in plain English. The AI generates a complete subnet design with reasoning, then offers to export to Terraform or visualize the result. Example prompts:
- "3-tier AWS app in /16 across 3 AZs"
- "Plan Kubernetes pod and service CIDRs for 5000 pods"
- "Allocate 10.0.0.0/16 to teams needing 500, 250, 100, 30 hosts"
Free tier includes 100 AI queries per month. Pro is unlimited.
Saving projects
Free users get 3 saved projects. Pro is unlimited.
Every project includes: starting network, subnet tree, notes, color tags, cloud mode, and history. You can export the full project as JSON at any time.
IPAM tracker
The lightweight IP Address Management module tracks subnet allocations, utilization, owners and conflicts. Use cases:
- Know which CIDRs are allocated vs reserved vs free
- Get alerts when a subnet exceeds 80% utilization
- Detect overlapping ranges before they cause routing problems
- Find the next free /24 in a parent network
Keyboard shortcuts
| Action | Shortcut |
|---|---|
| Focus CIDR input | / |
| Calculate | Enter |
| Toggle theme | Cmd/Ctrl + Shift + L |
| Open calculator | G then C |
| Open VLSM | G then V |
| Save project | Cmd/Ctrl + S |
| Share link | Cmd/Ctrl + Shift + S |
FAQ
Does the calculator work offline?
Yes. Once loaded, all calculation logic runs in your browser. You can install the site as a PWA for full offline use.
Why does my calculation differ from AWS console?
You probably forgot to switch to AWS cloud mode. AWS reserves 5 IPs per subnet, not the standard 2.
Is there a CLI?
Not yet. We're focused on the web experience first. Developer tools (CLI, SDKs, API) are on the roadmap.
Support
Found a bug or have a feature request? Email hello@cidrcalculator.net or file an issue on GitHub.
Pro and Team plans include priority email support (24h response).