VLSM Calculator

Allocated 3 subnet(s) from 192.168.0.0/24.

How it works

• Sorted the requested host counts from largest to smallest.
• For each requirement, chose the smallest prefix whose usable host count fits.
• Aligned the block to its size and advanced the allocation cursor.
• Reported the remaining space starting at 192.168.0.224.

block size = 2^(32 − prefix); usable = block size − 2

• The largest requirement is allocated first, which is what makes the plan fit as often as possible.
• A requirement larger than the base block is reported as an error rather than silently dropped.

Methodology

1. Parse the base CIDR block and the list of required host counts.
2. Sort requirements descending and choose the smallest prefix that fits each.
3. Align each block to its size, advance the cursor and detect exhaustion.
4. Report every segment plus the remaining address space.

Usage

1. Enter the base CIDR block.
2. List one required host count per line (or comma separated).
3. Run the allocation and copy the plan.

Examples

These are examples, not current values: they use fixed inputs, while the interactive tool computes live results in your browser.

Three departments in a /24

{
  "base": "192.168.0.0/24",
  "requirements": "100\n50\n20"
}

Two point-to-point links and a /25

{
  "base": "10.0.0.0/24",
  "requirements": "120,2,2"
}

Limitations

Why VLSM exists

A single subnet size wastes address space when a site has very different needs: a transit link needs two addresses while a user LAN might need hundreds. VLSM lets each segment choose its own prefix so the address plan fits the requirements.

Allocating the largest block first is the common heuristic: it keeps the alignment simple and leaves the most room for the smaller blocks that follow.