FLSM Calculator
Split a network into equal-sized fixed-length subnets and list each subnet's address range.
About FLSM Calculator
FLSM (Fixed-Length Subnet Masking) divides a network into equal-sized subnets by borrowing enough bits from the host portion to create the requested number of subnets. All subnets get the same prefix length and the same number of host addresses, which simplifies routing but may waste addresses if subnet sizes vary.
Formulas
Bits to borrow
bits = ceil(log2(subnet_count))
- n
- Number of subnets needed
New prefix
new_prefix = original_prefix + bits_borrowed
- bits_borrowed
- Number of host bits repurposed for subnetting
Worked example
With the default values below, FLSM Calculator returns Subnet Prefix Length of /18. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Network Address
- 10.0.0.0
- Network Prefixaccepts 0–30
- 16
- Number of Subnets Neededaccepts 2–1,024
- 4
Results
- Subnet Prefix Length
- /18
- Subnets Created
- 4
- Usable Hosts per Subnet
- 16,382
- Subnet List (first 8)
- 10.0.0.0/18 (10.0.0.1 - 10.0.63.254) 10.0.64.0/18 (10.0.64.1 - 10.0.127.254) 10.0.128.0/18 (10.0.128.1 - 10.0.191.254) 10.0.192.0/18 (10.0.192.1 - 10.0.255.254)
Step by step
- 1Bits needed for subnets= 2
- 2New prefix length= /18
- 3Actual subnets created= 4
- 4Hosts per subnet= 16382
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 whole numbers, 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
What is the difference between FLSM and VLSM?
FLSM creates equal-sized subnets (all get the same prefix length), while VLSM allows variable-sized subnets tailored to each segment's actual host count — VLSM wastes fewer addresses but requires a routing protocol that supports classless addressing.
Why might more subnets be created than requested?
Subnet counts must be powers of 2 (since you borrow whole bits). Requesting 5 subnets requires borrowing 3 bits, creating 8 subnets — the next power of 2 at or above the requested number.