About the VLSM Calculator
VLSM (Variable Length Subnet Masking) divides one address block into subnets of different sizes, so a network needing 100 hosts, two /30 point-to-point links, and a 20-host management LAN doesn't waste space on uniform subnets. This calculator takes your block and per-subnet host requirements and produces an optimal, non-overlapping allocation.
The algorithm matters: allocating largest-first prevents fragmentation, and each subnet must start on a boundary aligned to its own size. Doing this by hand invites overlaps — exactly the class of error that surfaces months later as an unroutable subnet.
Frequently asked questions
Why allocate the largest subnets first?
Subnet boundaries must align to the subnet's own size — a /25 can only start at .0 or .128. Allocating small subnets first scatters them across alignment boundaries and fragments the space, so a later large request won't fit even though enough total addresses remain. Largest-first packs perfectly.
How do I size a subnet for N hosts?
Find the smallest power of two that is ≥ N + 2 (network + broadcast). 50 hosts needs 64 addresses (/26); 30 hosts fits exactly in a /27 (32 addresses, 30 usable). Leave growth headroom deliberately — resizing a production subnet later is far more disruptive than over-allocating now.
Is VLSM still relevant in cloud VPCs?
Very — it is just called subnet planning now. Carving a VPC /16 into public, private, and database subnets per AZ is a VLSM exercise, and cloud reservations change the math (AWS reserves 5 IPs per subnet). EKS/GKE pod ranges consume far more address space than expected, making careful allocation the difference between smooth scaling and VPC renumbering.
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