Variable Length Subnet Masking allows different subnet masks within the same network, maximizing IP address utilization.

Why sort subnets by host count descending?

Sorting from largest to smallest minimizes address fragmentation and ensures the biggest subnets get contiguous blocks first.

What is CIDR block size?

CIDR block size = 32 - ceil(log2(hosts + 2)). The +2 accounts for network and broadcast addresses.

When does VLSM allocation fail?

When the total address space required by all subnets exceeds the capacity of the parent network.

How is VLSM different from FLSM?

VLSM uses different subnet mask lengths per subnet; FLSM (Fixed Length) uses the same mask for all subnets, wasting addresses on small subnets.

Can I export VLSM results?

Yes. Click Copy Markdown for a formatted table, or Download CSV to open in Excel.

VLSM Calculator with Steps

Variable Length Subnet Masking — allocate subnets based on actual host requirements. VLSM saves IP addresses by using different subnet masks within the same network, unlike fixed-length subnetting which wastes address space on small subnets. Enter a base network, define your subnet requirements by name and host count, then click Calculate VLSM to see the step-by-step allocation.

Base Network

Enter the parent network in IP/CIDR format

Quick Presets

📋 Bulk Paste Mode

Format: Name Hosts (one per line)

Subnet Requirements

How VLSM Works

Sort by hosts descending — largest first to minimize fragmentation.
Compute minimum CIDR — CIDR = 32 - ceil(log₂(hosts + 2)).
Allocate at next boundary — each subnet starts at address divisible by block size.
Verify no overflow — total ≤ base network capacity.
Review results — table shows all parameters for each subnet.

Read the full VLSM tutorial →