FLSM Calculator
Split a network into equal-sized fixed-length subnets and list each subnet's address range.
Inputs
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
Bits needed for subnets
ceil(log2(4))
= 2
New prefix length
16 + 2
= /18
Actual subnets created
2^2
= 4
Hosts per subnet
2^(32-18) - 2
= 16382
How it works
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
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.