CIDR Calculator & Subnet Planner

Calculate CIDR ranges, subnet masks, usable IPs, hosts, gateways, IPv4/IPv6 subnets, CIDR splits, and Terraform cidrsubnet examples.

Output:A ready-to-use configuration file for CIDR Calculator & Subnet Planner with best practices applied.

Quick Summary

Use a CIDR Calculator & Subnet Planner when you need to calculate subnet masks, usable IPs, host counts, IP ranges, subnet splits, and Terraform cidrsubnet examples for AWS VPCs or local networks.

What is this tool?

CIDR (Classless Inter-Domain Routing) notation is a compact way to specify an IP address and its associated routing prefix (e.g., 192.168.1.0/24). The number after the slash represents how many bits are fixed for the network, determining how many IP addresses are left for hosts.

A CIDR Calculator translates this slash notation into human-readable data: the subnet mask, the first and last usable IP addresses, the broadcast address, and the total host count. It is an essential tool for designing cloud architectures and preventing IP overlaps.

How to Use This Tool

Input the Base CIDR: Enter an IP address with its CIDR block (e.g., 10.0.0.0/16).
Calculate Subnet Data: The tool instantly calculates the network mask, broadcast address, and total usable host IPs.
Split Subnets (Optional): Select a smaller subnet size (e.g., /24) to split the parent block into multiple child subnets.
Review the List: View the generated list of subnets to assign them to your VPC availability zones.
Export: Download the subnet list as CSV or copy the Terraform cidrsubnet() helper function.

What This Tool Generates

CIDR network ranges (Start to End IP)
Subnet masks (e.g., 255.255.255.0)
Usable IP address counts
Lists of split subnets
Terraform cidrsubnet() interpolation examples
CSV exports of your network topology

Example Output Explanation

A basic calculation for the CIDR block 10.0.0.0/24:

Network:      10.0.0.0
Subnet Mask:  255.255.255.0
First Usable: 10.0.0.1
Last Usable:  10.0.0.254
Broadcast:    10.0.0.255
Usable IPs:   254

Best Practices

AWS VPC Subnet Planning: AWS reserves the first four and the last IP address in every subnet (e.g., network, router, DNS, future use, and broadcast). Therefore, a /28 subnet in AWS only gives you 11 usable IPs, not 14.
Terraform cidrsubnet Calculator: When using Infrastructure as Code, don't hardcode subnets. Use the cidrsubnet(prefix, newbits, netnum) function so your topology can scale dynamically without causing merge conflicts.
IPv4 vs IPv6: IPv6 uses 128-bit addresses and /64 is the standard subnet size for a single LAN. Our tool supports the massive integer math required for IPv6 splits.

Common Mistakes

Over-provisioning /16s: Assigning a massive /16 (65,536 IPs) to a single VPC is common, but assigning a /16 to a single subnet is dangerous. Always split your VPC into smaller /24s or /22s across multiple availability zones.
Overlapping CIDR Blocks: If you plan to connect two VPCs via peering or Transit Gateway, their CIDR blocks must not overlap. Always use a CIDR planner to map out your global network architecture before deploying.
Misunderstanding /32: A /32 means all 32 bits are fixed. It represents exactly one single IP address, typically used in firewall allow lists, not for networking subnets.

Security Notes

Do not publicly expose your internal VPC topology. While 10.x.x.x addresses are private, knowing your exact subnet structure helps attackers map your network if they breach the perimeter.
Use strict CIDR blocks (/32) when configuring firewall rules or database access to enforce the principle of least privilege.
Verify provider-specific reserved IP rules before finalizing your subnet sizes, as Azure, GCP, and AWS all have slight variations in how many IPs they reserve per block.

Testing Instructions

Validate overlaps using Terraform: 'terraform plan' will often catch overlapping subnets if you use the cidrsubnet() function properly.
Run 'ip addr show' on Linux to verify the actual CIDR applied to your network interface.

Frequently Asked Questions

What is a CIDR Calculator?

It is a network engineering tool that translates CIDR notation (like 192.168.1.0/24) into a subnet mask, broadcast address, and usable IP range.

How do I calculate CIDR subnet mask?

The number after the slash dictates the mask. A /24 means 24 bits are 1s, which translates to 255.255.255.0 in decimal format.

How many hosts are in a CIDR block?

The formula is 2^(32 - CIDR) - 2. For a /24, it is 2^(32-24) - 2 = 254 usable hosts. (The -2 accounts for the network and broadcast addresses).

How do I split CIDR into subnets?

Use our Subnet Splitter tool. To split a /16 into /24s, the tool adds 8 bits to the network mask, creating 256 distinct /24 child subnets.

What is usable address range?

It is the range of IPs you can actually assign to servers or devices. It excludes the very first IP (the network address) and the very last IP (the broadcast address).

What is /8 CIDR notation?

A /8 is a massive network block containing over 16.7 million IP addresses. The 10.0.0.0/8 block is commonly used as the foundation for large enterprise private networks.

Can I calculate IPv6 CIDR?

Yes, IPv6 CIDR works the same way but with 128 bits. The standard LAN subnet is a /64, which contains 18 quintillion addresses.

How do I use Terraform cidrsubnet?

cidrsubnet('10.0.0.0/16', 8, 2) takes a /16, adds 8 network bits (making it a /24), and returns the 3rd subnet (index 2): 10.0.2.0/24.

How We Keep Your Configs Safe & Valid

Built-in Error Checking

Every file is checked against official rules. We catch missing fields and bad syntax. YAML indentation errors are flagged right away. Kubernetes, Terraform, and Docker specs are all covered. API versions and labels are verified too. You get valid output every time you generate.

100% Private & Local

All tools run in your browser only. Your API keys never leave your machine. We do not use any tracking scripts. No data is sent to any server. Passwords and secrets stay on your device. Crypto operations use the Web Crypto API. Your privacy is fully protected at all times.

Secure Settings by Default

Configs use safe defaults out of the box. Containers run as non-root users. Root filesystems are set to read-only. Dangerous Linux capabilities are dropped. Network policies limit pod-to-pod traffic. TLS 1.3 is enabled for web servers. Security headers are added where needed.

Ready for CI/CD & Git

Output files are ready for your Git repo. Use them with ArgoCD, Flux, or GitHub Actions. Files use clear formatting and comments. Code review is easy for your team. Indentation and key order are consistent. Test in staging before going to production. Every file is clean and well-structured.

Infrastructure as Code

Store configs in Git alongside your code. Terraform modules include typed variables. Backend configs support remote state locking. Outputs work across multiple modules. Ansible playbooks use clear task steps. Chef and Puppet configs are also supported. Every file works with version control tools.

Monitoring & Tracing

Set up Prometheus with auto-discovery rules. Create Grafana dashboards with template variables. Add alerting rules with severity labels. Use OpenTelemetry for trace collection. Forward logs to Loki or Elasticsearch. Connect to Jaeger or Tempo for tracing. Monitor metrics, logs, and traces together.

Container & Docker Safety

Dockerfiles use multi-stage builds for small images. Base images are pinned to exact versions. Dev files are excluded from final images. Health checks are added for orchestrator use. Containers switch to non-root users. Docker Compose uses named volumes and networks. Resource limits are set in deploy configs.

Multiple Output Formats

Export as YAML, JSON, HCL, or TOML. Kubernetes uses YAML with proper separators. Terraform uses HCL with correct escaping. JSON output has consistent indentation. Copy to clipboard with one click. Preview output with syntax highlighting. Line numbers help you review quickly.