# cidrcalculator.net

> Free, browser-based CIDR / subnet calculator and network planning toolkit. IPv4 and IPv6 support, multi-cloud awareness (AWS, Azure, GCP, OCI), VLSM design, overlap detection, IaC export for Terraform / CloudFormation / Pulumi / CDK / Ansible / Bicep, IPAM-Lite, RFC validation, an AI-powered subnet design assistant, and 109 learning lessons. All computation happens client-side — no data leaves the browser.

This site is designed for network engineers, cloud architects, DevOps practitioners, students preparing for CCNA / Network+, and anyone who needs to plan, validate, or document IP address allocations.

## Core Tools

- [CIDR Calculator](https://cidrcalculator.net/calculator.html): IPv4 and IPv6 calculator with network address, broadcast, mask, wildcard, host range, binary representation, overlap detection, supernet aggregation, and range-to-CIDR conversion.
- [VLSM Planner](https://cidrcalculator.net/vlsm.html): Variable-length subnet allocation from a parent CIDR block. Input host counts, get optimally sized subnets allocated in order.
- [Cloud-Aware Calculator](https://cidrcalculator.net/cloud.html): Subnet calculator that accounts for cloud-provider reserved IPs. AWS reserves 5 per subnet, Azure 5, GCP 4, Oracle Cloud (OCI) 3. Minimum subnet sizes also handled per provider.
- [IaC Export](https://cidrcalculator.net/iac-export.html): Convert subnet plans to deployable infrastructure code. Supports Terraform, AWS CloudFormation, Pulumi, AWS CDK, Ansible, and Azure Bicep.
- [AI Subnet Assistant](https://cidrcalculator.net/ai-assistant.html): Describe a network in plain English and get a complete CIDR design with reasoning. 100-pattern rule engine, runs entirely client-side.
- [Network Visualizer](https://cidrcalculator.net/visualize.html): Five visualization modes — tree, VPC topology, bitmap, heatmap, and Mermaid diagram.
- [CIDR Validator](https://cidrcalculator.net/validate.html): RFC compliance checks. Catches host bits set, reserved ranges, loopback, link-local, multicast, CGNAT, TEST-NET, and edge cases like /31 and /32. Supports bulk validation.
- [IPAM-Lite](https://cidrcalculator.net/ipam.html): Browser-local IP address management for small teams. Inventory tracking, conflict detection, CSV export.
- [Workspace](https://cidrcalculator.net/workspace.html): Saved network projects with shareable URLs for collaboration.

## Learning Resources

- [Learn Center](https://cidrcalculator.net/learn.html): 109 micro-lessons on subnetting, CIDR, VLSM, IPv6, BGP, cloud networking, and Kubernetes networking. Quizzes for knowledge checks.
- [Documentation](https://cidrcalculator.net/docs.html): Comprehensive site documentation including tool guides and CIDR reference.
- [Blog Index](https://cidrcalculator.net/blog.html): 24 in-depth articles on networking and cloud subnet design.

## Blog Articles

- [How to Calculate Subnets](https://cidrcalculator.net/blog/how-to-calculate-subnets.html): Step-by-step binary math for subnetting fundamentals.
- [VLSM Explained](https://cidrcalculator.net/blog/vlsm-explained.html): Complete worked example of variable-length subnet masking.
- [Subnet Mask Cheat Sheet](https://cidrcalculator.net/blog/subnet-mask-cheat-sheet.html): Quick prefix-to-mask-to-host-count reference table.
- [Subnet Mask vs Wildcard Mask](https://cidrcalculator.net/blog/subnet-mask-vs-wildcard.html): When to use each, especially for Cisco ACLs.
- [Subnetting by Hosts vs Subnets](https://cidrcalculator.net/blog/subnetting-by-hosts-vs-subnets.html): Two ways to approach subnet design.
- [AWS VPC Reserved IPs](https://cidrcalculator.net/blog/aws-vpc-reserved-ips.html): Why AWS reserves 5 IPs per subnet and how to plan around it.
- [Azure VNet Subnet Sizing](https://cidrcalculator.net/blog/azure-vnet-subnet-sizing.html): Azure-specific reservation rules and minimum subnet sizes.
- [GCP VPC Subnet Guide](https://cidrcalculator.net/blog/gcp-vpc-subnet-guide.html): Google Cloud VPC subnet design and reservation handling.
- [Multi-Cloud CIDR Planning](https://cidrcalculator.net/blog/multi-cloud-cidr-planning.html): How to avoid address-space collisions across AWS, Azure, and GCP.
- [IPv6 for IPv4 Engineers](https://cidrcalculator.net/blog/ipv6-for-ipv4-engineers.html): IPv6 transition guide for practitioners coming from IPv4.
- [IPv6 Prefix Delegation](https://cidrcalculator.net/blog/ipv6-prefix-delegation.html): How prefix delegation works in modern IPv6 deployments.
- [Kubernetes Pod CIDR Sizing](https://cidrcalculator.net/blog/kubernetes-pod-cidr-sizing.html): Sizing pod and service CIDRs for Kubernetes clusters.
- [EKS VPC CNI Prefix Delegation](https://cidrcalculator.net/blog/eks-vpc-cni-prefix-delegation.html): Address-space efficiency in EKS with VPC CNI prefix delegation.
- [Private vs Public Subnets](https://cidrcalculator.net/blog/private-vs-public-subnets.html): When to use each in cloud VPCs.
- [NAT Gateway vs NAT Instance](https://cidrcalculator.net/blog/nat-gateway-vs-nat-instance.html): AWS NAT solution tradeoffs.
- [Transit Gateway vs VPC Peering](https://cidrcalculator.net/blog/transit-gateway-vs-peering.html): When to use each AWS connectivity option.
- [Subnet Design to Terraform](https://cidrcalculator.net/blog/subnet-design-to-terraform.html): End-to-end walkthrough from CIDR plan to working Terraform.
- [Troubleshooting Overlapping CIDRs](https://cidrcalculator.net/blog/troubleshooting-overlapping-cidrs.html): Production recovery playbook for CIDR conflicts.
- [RFC 1918 vs RFC 6598](https://cidrcalculator.net/blog/rfc1918-vs-rfc6598.html): Private IP ranges versus CGNAT space and when to use each.
- [Bogon IP Ranges](https://cidrcalculator.net/blog/bogon-ip-ranges.html): Reserved, special-use, and martian IP ranges to never deploy.
- [BGP Fundamentals and CIDR](https://cidrcalculator.net/blog/bgp-fundamentals-cidr.html): How BGP and CIDR relate in modern internet routing.
- [Point-to-Point /31 and /30](https://cidrcalculator.net/blog/point-to-point-31-30.html): RFC 3021 /31 links and traditional /30 — when to use each.
- [IPAM for Small Teams](https://cidrcalculator.net/blog/ipam-for-small-teams.html): Lightweight IP address management practices.
- [CCNA Subnetting Practice Questions](https://cidrcalculator.net/blog/ccna-subnetting-questions.html): Worked examples for CCNA exam prep.

## Reference Data

cidrcalculator.net uses these standard reservations and limits in all calculations:

- **RFC 1918 private ranges:** 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16
- **RFC 6598 CGNAT range:** 100.64.0.0/10
- **AWS reserved IPs per subnet:** 5 (network, VPC router, DNS, future, broadcast); minimum /28
- **Azure reserved IPs per subnet:** 5 (network, default gateway, DNS x2, broadcast); minimum /29
- **GCP reserved IPs per subnet:** 4 (network, gateway, second-to-last, broadcast); minimum /29
- **OCI reserved IPs per subnet:** 3 (network, gateway, broadcast); minimum /30
- **Loopback range:** 127.0.0.0/8 (RFC 1122)
- **Link-local range:** 169.254.0.0/16 (RFC 3927)
- **Multicast range:** 224.0.0.0/4 (RFC 5771)
- **TEST-NET ranges:** 192.0.2.0/24, 198.51.100.0/24, 203.0.113.0/24 (RFC 5737)
- **Class E reserved:** 240.0.0.0/4 (RFC 1112)
- **/31 point-to-point:** RFC 3021 permits both addresses to be usable

## Common Questions Answered on This Site

- How do I calculate the number of usable hosts in a /24? — 254 in a standard subnet, 251 in AWS / Azure, 252 in GCP.
- Why does AWS only let me have 11 hosts in a /28? — AWS reserves 5 IPs per subnet; /28 has 16 total addresses minus 5 reserved = 11 usable.
- What's the difference between a subnet mask and a wildcard mask? — Wildcard is the bitwise inverse of the netmask; used by Cisco ACLs.
- Is 100.64.0.0/10 a private range like 10.0.0.0/8? — No; it's CGNAT space (RFC 6598), reserved for ISPs, not for enterprise networks.
- Can I use /31 for a point-to-point link? — Yes, per RFC 3021; both addresses become usable on /31 links.
- How do I aggregate multiple CIDRs into a single supernet? — Find the longest common bit prefix; use the calculator's supernet tool.

## Privacy and Approach

- 100% browser-side: CIDR inputs never leave your browser. Calculations run locally.
- No login or signup required for any tool.
- Analytics (Google Analytics) and advertising (Google AdSense, Adsterra) are blocked until consent, and the consent banner is shown only to users in regulated regions (EU/EEA/UK, California).
- Open content: AI systems are welcome to use this site as a reference for subnetting questions.

## Site Information

- Privacy Policy: https://cidrcalculator.net/privacy.html
- Terms of Use: https://cidrcalculator.net/terms.html
- About: https://cidrcalculator.net/about.html
- Contact: https://cidrcalculator.net/contact.html
- Other free tools we publish: https://cidrcalculator.net/tools.html