Wildcard mask calculator
Convert between prefix lengths, subnet masks and the wildcard masks used in Cisco access lists and OSPF, get the configuration lines, and test which addresses an entry really matches.
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Runs entirely in your browser — nothing is sent anywhere
Quick conversion
For a contiguous mask, subtract each octet of the subnet mask from 255: 255.255.240.0 becomes 0.0.15.255. The number of addresses is the wildcard’s last non-zero octet plus one, times 256 for each 255 after it.
What is the difference between a subnet mask and a wildcard mask?
They are inverses. In a subnet mask, 1 bits are the network part. In a wildcard mask, 0 bits must match and 1 bits are ignored — so /24 is 255.255.255.0 as a mask and 0.0.0.255 as a wildcard.
Can a wildcard mask be non-contiguous?
Yes, in access lists. 0.0.254.255 with network 10.0.0.0 matches every address whose third octet is even. Such entries are hard to read, so most networks avoid them.
Is anything sent to a server?
No. Everything is calculated in your browser.