Security

CIDR Subnet Calculator

Inspect an IPv4 CIDR block and calculate its subnet mask, network address, broadcast address, and usable host range.

Last reviewed: April 30, 2026Free toolMethodology

CIDR Subnet Calculator

These fields start with sample inputs. Keep them or replace them, then run the tool to show a fresh result.

Result

Calculating the sample result.

Why it matters

CIDR notation is efficient but opaque at a glance, so operators and developers need a way to translate it into practical network boundaries.

When to use

  • Planning VPC or subnet layouts
  • Checking firewall and allowlist ranges
  • Verifying how many usable IPs a subnet provides

Inputs & Outputs

Inputs

  • CIDR input should be a standard IPv4 address followed by a slash prefix, such as 10.0.4.0/24.

Outputs

  • Network details show the subnet mask, network address, and broadcast address.
  • Usable host range and host count show what addresses are actually available for assignment.

CIDR expansion method

The tool converts the IPv4 address and prefix into a bitmask, derives the network and broadcast boundaries, and calculates the usable host range.

CIDR prefix length determines how many high-order bits are fixed for the network

Worked example

1

Subnet planning

A team wants to inspect 10.0.4.0/24 before carving ranges for an environment.

Inputs

  • CIDR: 10.0.4.0/24

Steps

  • Apply the prefix mask
  • Derive network, broadcast, and host range

Result

  • The tool shows the subnet boundaries and usable host count clearly.

Edge cases & caveats

  • This tool focuses on IPv4 rather than IPv6 subnet math.
  • Cloud providers may reserve some addresses inside otherwise usable host ranges.

Frequently Asked Questions

Why is the usable host count smaller than the total address count?

Because the network and broadcast addresses are reserved in standard IPv4 subnetting.

Can I use this for cloud subnets too?

Yes, but remember that some cloud providers reserve extra addresses inside each subnet.

Keep going

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