Subnet Mask Validity Check
Enter a subnet mask and it checks whether it is valid (contiguous 1-bits) and shows the prefix, wildcard, and host count. All checks happen inside your browser.
Enter a subnet mask.
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CIDR prefix-
Wildcard-
Total addresses-
How to use
- Enter the subnet mask to check (e.g. 255.255.255.0).
- Click "Check".
- Check the validity, prefix, and wildcard.
Frequently asked questions
What makes a subnet mask valid?
The 1-bits must be filled contiguously from the left, followed by all 0-bits. For example, 255.255.255.0 is valid but 255.0.255.0 is invalid.
Why is a value like 255.0.255.0 invalid?
Because a 0-bit is inserted between 1-bits, breaking the contiguity rule. A subnet mask must have its 1-bits run without a break.
Are the entered values sent to a server?
No. All checks happen only inside your browser, and the values are never sent to or stored on any server.