VLSM Subnet Calculator
Optimize your network addressing plan by creating subnets of varying sizes to fit specific host requirements.
Enter host requirements to calculate variable subnets.
What this tool does
The Variable Length Subnet Mask (VLSM) calculator performs recursive subnetting to break a major network block into smaller, unequal-sized subnets based on specific host requirements. It optimizes address space usage by assigning the smallest possible CIDR block that satisfies the host count for each segment. This reduces wasted IP addresses compared to fixed-length subnetting.
When engineers use this
- Designing IP addressing plans for multi-site networks with varying user densities.
- Allocating /30 or /31 subnets for point-to-point links while reserving larger blocks for LANs.
- Maximizing the efficiency of a limited public IP allocation from an ISP.
- Structuring hierarchical addressing schemes to facilitate efficient route summarization.
How to interpret the results
- Allocated Subnet: The calculated network address and mask that fits the requested host count.
- Cap (Capacity): The total number of usable hosts supported by the assigned subnet mask.
- Unused Space: Remaining address blocks that are available for future allocation.
Why use VLSM?
Variable Length Subnet Masking (VLSM) is crucial for conserving IP addresses. Unlike FLSM (Fixed Length Subnet Masking), which forces all subnets to be the same size regardless of need, VLSM allows you to create subnets that fit the exact number of hosts required. However, FLSM is still useful for simple, uniform network designs (e.g., assigning a /24 to every site).
VLSM vs CIDR
While often used interchangeably, CIDR (Classless Inter-Domain Routing) refers to the notation and aggregation of routes on the internet, whereas VLSM specifically refers to the technique of sub-dividing a network into unequal parts.