VLSM subnetting in plain language
VLSM (Variable Length Subnet Masking) lets you carve one address block into subnets of different sizes. Traditional fixed-length subnetting forces every subnet to match the largest need, wasting addresses on small segments such as router links. This VLSM calculator removes the guesswork: list your segments, and it allocates the right block to each.
How the VLSM calculator works
- Sort the segments from the most hosts to the fewest.
- For each, add 2 for the network and broadcast addresses, then round up to the next power of two.
- Convert the block size to a prefix (64 addresses = /26).
- Place each block on its natural boundary, directly after the previous one.
- Report the remaining free blocks.
VLSM calculation example
With 192.168.1.0/24, 60 + 25 + 12 + 2 hosts fit into a /26, a /27, a /28 and a /30, using 116 of 256 addresses and leaving a /29 and a /25 free — a fixed-length design would have needed four /26 subnets and left nothing spare. The full worked table is in the VLSM guide.
VLSM vs CIDR vs FLSM
CIDR is the classless addressing system; VLSM is the technique of using several prefix lengths inside your own CIDR block; FLSM is the older equal-size approach. See VLSM vs CIDR and VLSM vs FLSM for a side-by-side comparison, and the subnetting guide if you are new to the basics.