Question 1

What is a subnet calculator?

Accepted Answer

A subnet calculator is a tool that helps network administrators and engineers divide IP networks into smaller, manageable segments called subnets. It calculates important network information such as network address, broadcast address, first and last usable host addresses, subnet mask, wildcard mask, total and usable host counts, and CIDR notation. This tool is essential for IP address management, network planning, and optimizing resource allocation in both on-premises and cloud environments.

Question 2

How do I calculate a subnet mask?

Accepted Answer

To calculate a subnet mask, determine your requirements (how many subnets and hosts per subnet needed), calculate bits needed using the 2^n formula, and create the mask with consecutive 1s (network portion) followed by 0s (host portion). For example, a /24 CIDR notation equals subnet mask 255.255.255.0, providing 256 total addresses (254 usable hosts). Our subnet calculator automates this process and handles cloud provider-specific IP reservations automatically.

Question 3

What is CIDR notation?

Accepted Answer

CIDR (Classless Inter-Domain Routing) notation is a compact method for specifying IP addresses and their associated network masks. It uses a forward slash followed by a number (e.g., /24) that indicates how many bits represent the network portion. Common examples: /8 = 16,777,216 addresses, /16 = 65,536 addresses, /24 = 256 addresses, /28 = 16 addresses, /32 = 1 address (host route). CIDR replaced classful networking, allowing more flexible IP address allocation.

Question 4

How many usable hosts in a /24 subnet?

Accepted Answer

A /24 subnet provides 254 usable host addresses in standard networking. Total addresses: 2^8 = 256, minus 1 network address and 1 broadcast address = 254 usable. Cloud providers differ: AWS VPC has 251 usable hosts (reserves 5 IPs), Azure VNet has 251 usable hosts, and Google Cloud has 252 usable hosts (reserves 4 IPs). Our calculator automatically adjusts for these cloud provider reservations.

Question 5

What IP addresses does AWS reserve in a VPC subnet?

Accepted Answer

AWS reserves 5 IP addresses in every VPC subnet: Network address (first IP), VPC Router (second IP), DNS Server (third IP), Future Use (fourth IP), and Broadcast address (last IP). For a /24 subnet with 256 total addresses, this leaves 251 usable IP addresses for EC2 instances and other resources. Our AWS VPC mode automatically calculates these reservations.

Question 6

What IP addresses does Azure reserve in a VNet subnet?

Accepted Answer

Azure reserves 5 IP addresses in every Virtual Network subnet: Network address (first IP), Default gateway (second IP), DNS mapping IPs (third and fourth IPs for Azure DNS mapped to VNet space), and Broadcast address (last IP). For a /24 subnet, this leaves 251 usable IP addresses for VMs, App Services, and other Azure resources. Azure requires a minimum /29 subnet (8 addresses) to accommodate these reservations, leaving 3 usable IPs.

Question 7

What IP addresses does Google Cloud reserve in a VPC subnet?

Accepted Answer

Google Cloud reserves 4 IP addresses in every VPC subnet: Network address (first IP), Default gateway (second IP), Second-to-last IP (reserved by GCP), and Broadcast address (last IP). For a /24 subnet, this leaves 252 usable IP addresses—one more than AWS or Azure. GCP's minimum subnet size is /29 (8 addresses, 4 usable) and maximum is /8 for custom mode VPCs. Our Google Cloud mode calculates these reservations automatically.

Question 8

What's the difference between IPv4 and IPv6 subnetting?

Accepted Answer

IPv4 uses 32-bit addresses (4.3 billion total) with CIDR /0 to /32, subnet masks like 255.255.255.0, and has broadcast addresses. IPv6 uses 128-bit addresses (340 undecillion total) with CIDR /0 to /128, prefix length notation only, no broadcast addresses (uses multicast), and standard /64 prefixes for most networks. Cloud providers typically assign /64 prefixes for IPv6 subnets, providing 18 quintillion addresses per subnet.

Question 9

How do I split a subnet into smaller networks?

Accepted Answer

To split a subnet, start with your parent subnet (e.g., 10.0.0.0/16), increase the CIDR prefix to create smaller subnets. Each +1 to the prefix halves the network size. For example, splitting 10.0.0.0/16 into 2 networks creates 10.0.0.0/17 and 10.0.128.0/17. Our Subnet Management feature lets you visually split subnets with a single click, choose custom CIDR sizes, join adjacent subnets, and export your subnet plan to various formats.

Question 10

Is this subnet calculator free to use?

Accepted Answer

Yes! This subnet calculator is completely free and open source. Features include IPv4 and IPv6 subnet calculations, AWS/Azure/Google Cloud modes, subnet splitting and joining, save and load configurations locally, export to Terraform/CloudFormation/Bicep for IaC, export to CSV/JSON/text formats, keyboard shortcuts, dark and light themes, and no registration required. We follow a privacy-first approach with no data collection, all calculations in your browser, no server-side processing, and open source code on GitHub.

Question 11

How do I save and load subnet configurations?

Accepted Answer

Our subnet calculator lets you save network configurations locally in your browser. Click the Save icon or press Ctrl+S to save with a name and description. Click Load or press Ctrl+O to browse and restore saved configurations. You can also export to JSON files for backup or sharing, and import files from other browsers. Auto-save protection saves your work every 30 seconds, with recovery offered on next visit. All data stays in your browser local storage—nothing is sent to any server.

Question 12

How do I export subnets to Terraform, CloudFormation, or Bicep?

Accepted Answer

Our subnet calculator generates Infrastructure as Code (IaC) for AWS, Azure, or Google Cloud. Supported formats include Terraform HCL, AWS CloudFormation YAML, and Azure Bicep. To export: configure your network, split into subnets, select the subnets you want, open the Export dropdown, and choose your IaC format. Exports include VPC/VNet resources, individual subnet resources with proper naming from your labels, cloud provider-specific best practices, and helpful comments. The generated code is ready to use with terraform apply or equivalent commands.

Professional Subnet Calculator