Subnet mask calculator

Reviewed by the CalculatorHub team, edited by James Graham. Method: a standard formula rather than a jurisdiction-specific rule. How we document and test our methods. Last verified 2026-07-09.

The Subnet mask calculator computes subnet mask from the relation mask from prefix length. It takes a single input (prefix length) and returns the subnet mask. These calculators are for network and systems administrators, software and hardware engineers, and computing students, turning a stated value, address, string or configuration into the exact figure, code or encoding that the rule behind it produces, without working it through by hand. Enter your values below and the result updates instantly, and you can share a permalink that pre-fills the exact calculation. Check the base, unit and width of each input before entering it: hexadecimal against decimal, bits against bytes, binary against decimal prefixes, and 0-based against 1-based positions are the usual sources of a wrong answer here, and a result that is out by a neat factor is far more often one of those than an arithmetic slip. For example, with prefix length = 16, the subnet mask works out to 255.255.0.0, and the worked example further down the page shows every step so you can follow the arithmetic and reproduce it by hand. This is a standard formula rather than a jurisdiction-specific rule, so it does not depend on an external rate or schedule, and the figure above each result carries the date it was last verified. The result follows the definition and the exact values you supply, so it can differ from what a live system reports: reserved and unusable values, protocol and filesystem overhead, the width and precision of the data type actually in use, and the conventions a particular language, device or display applies all sit outside the calculation, so confirm the figure against the environment it will be used in.