Subnet Split (VLSM) Calculator

Work out subnet split (vlsm) instantly with clear inputs, formula shown and shareable results.

About Subnet Split (VLSM) Calculator

Variable length subnet masking sizes each subnet to its own host count instead of forcing one mask on the whole network. Round the host requirement up to the next power of two after adding the network and broadcast addresses, and the number of host bits gives the child prefix directly. Comparing subnets needed against subnets available tells you whether the parent block is big enough.

Formula

VLSM child prefix

host bits = ceil(log2(hosts + 2)); child prefix = 32 - host bits; children = 2^(child prefix - base prefix)

Worked example

With the default values below, Subnet Split (VLSM) Calculator returns Child subnet prefix of 23. Change any input above and every figure here is recalculated in your browser.

Inputs used

Base block prefix (/n)accepts 1–30 bits: 16 bits
Hosts needed per subnetminimum 1 hosts: 500 hosts
Subnets neededminimum 1: 6

Results

Child subnet prefix: 23 /n
Usable hosts per child subnet: 510
Child subnets available in the block: 128
Base block utilisation: 4.60%

Step by step

1Values used
2VLSM child prefix
3Child subnet prefix= 23 /n
4Usable hosts per child subnet= 510
5Child subnets available in the block= 128
6Base block utilisation= 4.60

Assumptions and limits

The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Why add 2 before taking the logarithm?

Because the network and broadcast addresses are not assignable, a subnet that must hold 510 hosts needs 512 addresses, which is 9 host bits and therefore a /23.

How should VLSM subnets be ordered?

Allocate the largest subnets first from the top of the block. Filling large requirements after small ones fragments the space and forces you to waste an aligned block.