Updated: 11/16/2019 by Computer Hope
Illustration of a bitmask.

With computers, a mask is a special value that acts as a data filter. It's called a "mask" because it reveals some parts of digital information, and conceals or alters others.


In binary operations, a bitmask can filter bit values using logical operations. For instance, a bitmask of 00001111 one operand of the boolean AND operation, it converts the first four bits of the other operand to 0. The final four bits will be unchanged. This operation is called "masking out" the first four bits, changing them to 0.

If the OR operator is used, any 1 values in the bitmask produce a 1 in the corresponding result, and other bits remain unchanged. So, a bitmask of 00001111, used with OR, will "mask out" the last four bits, changing them to 1.

If the XOR operator is used, any 1s in the bitmask cause corresponding bits in the operand to be toggled1 becomes 0, and 0 becomes 1.

Bitmask examples
Bitmask 00001111 00001111 00001111 00001111
Operation AND AND OR XOR
Operand 11010010 01101101 10010110 01011010
Result 00000010 00001101 10011111 01010101


A netmask is another type of bitmask, used in computer networking. One type of netmask, a subnet mask, defines logical divisions ("subnets") of a computer network. For instance, a subnet mask of masks out the first three bytes of an IP address, leaving only the final byte — the host identifier.

Search masks

In Microsoft Windows, a search mask is a string, which may contain wildcards, which filters search results. It's commonly used to search for files by name. For instance, in the command:

forfiles /m "s*.exe"

The search mask s*.exe is used by the forfiles command to locate all .exe files in the current directory whose name begins with s.


In Unix-like operating systems, such as Linux, BSD (Berkeley Software Distribution), and macOS X, a umask is an octal value mask that defines the permissions of new files.

For information about user file creation masks in Linux, see umask in our Linux command guide.

Filter, Software terms