IP subnet (CIDR) calculator

When writing firewall rules, blocking an IP range or planning a local network, you need to know which addresses a CIDR block covers. The tool calculates network and broadcast, first and last usable address, mask and address type, and can split the block into smaller subnets.

How to use it

1. Enter the IP address with its prefix (e.g. 192.168.1.0/24) or a subnet mask.
2. Press the run button. Results are listed separately as definite problems, possible issues, unverifiable and passed; each finding shows what was found, what it means, the fix and the limits of the check.
3. Apply the suggested fix and run the tool again to confirm. Use “Copy result” to add the report to a support ticket.

Example

Before adding a range to cPanel's IP Blocker, confirm that 203.0.113.0/27 covers exactly 32 addresses. The “Example” button in the form fills in a scenario like this and shows the result immediately.

Scope and limits

The calculation runs in your browser; the address is sent nowhere.

Your inputs are not added to the address bar or passed to analytics, and result pages are not indexed by search engines.

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