CIDR Calculator

When this tool is a good fit

CIDR Calculator helps when you need network address, first and last addresses, masks, counts, and subnet previews from an IP address or network with a CIDR prefix. It is most useful for understanding IPv4 and IPv6 ranges during firewall changes, allowlists, subnet planning, and security scoping. The notes below focus on the settings that matter, the checks worth doing before you export, and the limits that tend to cause mistakes.

• Takes an IP address or network with a CIDR prefix.
• Lets you adjust IPv4 or IPv6 mode, prefix length, and whether to split into smaller subnets.
• Calls out what to review: range boundaries, address count, broadcast behavior for IPv4, and whether the scope is too broad.
• Produces network address, first and last addresses, masks, counts, and subnet previews.
• Fits common jobs such as understanding IPv4 and IPv6 ranges during firewall changes, allowlists, subnet planning, and security scoping.

Example jobs

• Calculate 192.168.1.10/24 to find the network address and broadcast address.
• Inspect 10.0.0.0/16 for total addresses and usable host range.
• Check 2001:db8::1/64 or fd00:1234::/48 to calculate IPv6 ranges and subnet previews.

How to use CIDR Calculator

1. Prepare an IP address or network with a CIDR prefix.
2. Set IPv4 or IPv6 mode, prefix length, and whether to split into smaller subnets before running the tool.
3. Generate or process the result, then review range boundaries, address count, broadcast behavior for IPv4, and whether the scope is too broad.
4. Copy, export, or download network address, first and last addresses, masks, counts, and subnet previews.
5. Run a second pass if the destination system, document, or workflow has stricter requirements.

What to know before you use it

• IPv4 addresses are calculated as 32-bit values and IPv6 addresses as 128-bit values in the browser.
• The tool derives network, range, address counts, subnet splits, and IPv4-only fields such as broadcast and wildcard mask where applicable.
• Subnet splitting previews the first 16 deterministic child networks for a larger target prefix.
• IPv4 calculations use 32-bit ranges and IPv6 calculations use 128-bit ranges with different conventions.
• IPv6 has no broadcast address, so IPv4-only fields should not be forced onto IPv6 results.

Checks and edge cases

• Verify ownership and intended scope before scanning, blocking, or allowlisting a CIDR range.
• Use smaller prefixes when documenting security scope to avoid accidental overreach.

How your data is handled

All CIDR and subnet calculations run in your browser. Toolumina does not upload, save, or log the IP ranges you enter.

FAQ