Subnet Mask Converter
Enter a subnet mask (e.g. 255.255.255.0) or CIDR prefix (e.g. /24) to convert between mask, CIDR, wildcard mask, and host count. All calculations happen inside your browser.
Enter a mask or CIDR to see the result.
How to use
- Enter either a subnet mask or a CIDR prefix.
- The CIDR, mask, wildcard, and host count are calculated automatically.
- Use it for router ACLs or subnet design.
Frequently asked questions
What can I enter?
Enter any of a subnet mask (e.g. 255.255.255.0) or CIDR prefix (e.g. /24 or 24), and it converts to the other formats.
What is a wildcard mask?
A wildcard mask is the bitwise inverse of the subnet mask, used mainly in router ACLs. For example, the wildcard for 255.255.255.0 is 0.0.0.255.
Are the values I enter sent to a server?
No. All calculations happen only inside your browser, and the values you enter are never sent to or stored on any server.
Practical example and verification criteria
Subnet Mask Converter: Distinguish IPv4 from IPv6 and select exactly the input the page accepts: an address, CIDR prefix, netmask, or start/end range. Each IPv4 octet must be 0–255 and a prefix must fit the address family's bit range. For host counts, first check the rules for including network and broadcast addresses.
Subnet Mask Converter: For a quick check, start with /24 ↔ 255.255.255.0 as a small input whose result relationship you can verify directly. Confirm that the input summary, units, and options match your intent before applying the tool to real values or a copy of the source; this reduces setup mistakes.
Subnet Mask Converter: Split the address at binary boundaries and verify that the first and last network addresses, membership result, and total address count agree. Feed a conversion result into the reverse tool to see whether the original is recovered. Remember that a calculation does not change router, VLAN, or firewall settings or guarantee the current route.
Subnet Mask Converter · Work record: For real work, record the input conditions, selected options, and result together. Do not assume the same state will remain after a refresh or device change. Preserve important results separately using copy, download, a screenshot, or another method the page supports, then reopen them in the target environment.
Subnet Mask Converter · Error check: If an error or unexpected result appears, reduce the input, run again with default options, and then change one condition at a time. Recording error messages and boundary values helps distinguish format issues, unsupported cases, and browser resource limits, and prevents an empty result from being mistaken for success.
Subnet Mask Converter · Scope of use: Check the page guidance for browser processing versus server lookup, storage location, and supported scope. Use sensitive source material or secrets only when strictly necessary. High-impact work such as legal, financial, or medical decisions or changes to an operational system requires current authoritative information and a separate verification process.