Variable Length Subnet Masking (VLSM) allows a network to be divided into subnets of different sizes. Unlike classful or fixed-length subnetting, each subnet gets only the address space it actually needs, making VLSM far more efficient for real-world networks.

Why does the tool sort subnets largest-first?

Allocating the largest subnets first from the base address avoids fragmentation and ensures smaller subnets always fall on valid boundaries. This is the standard VLSM algorithm taught in networking courses and used by network engineers.

How many hosts can a subnet actually hold?

A subnet with prefix /N has 2^(32-N) total addresses. Two are reserved — the network address and the broadcast address — leaving 2^(32-N) - 2 usable host addresses. For example, a /26 holds 64 total and 62 usable hosts.

What if my requirements exceed the base network capacity?

The calculator will show an error if the cumulative address space required for all subnets exceeds what the base network can provide. You will need to use a larger base network or reduce host requirements.

Can I use this for IPv6?

No. This tool is designed for IPv4 only. IPv6 subnetting uses 128-bit addresses and different conventions; a dedicated IPv6 tool is needed for those calculations.

What does the "Remaining Addresses" row mean?

After all subnets are allocated, the leftover addresses in the base network are shown as unallocated space. These can be used for future subnet additions without renumbering existing subnets.

VLSM (Variable Length Subnet Mask) Calculator

Name: VLSM (Variable Length Subnet Mask) Calculator
Availability: InStock
Author: Nham Vu

Enter a base network and a list of required host counts to get optimally allocated VLSM subnets with no wasted address space.

VLSM Configuration

Base Network (CIDR)

Must use the network address, not a host address.

Subnet Requirements

Enter the number of usable hosts needed per subnet.

Configure the base network and subnet requirements, then click Calculate.

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Summary

Enter a base network and a list of required host counts to get optimally allocated VLSM subnets with no wasted address space.

How it works

Enter the base network in CIDR notation (e.g. 192.168.10.0/24).
Add one or more subnet requirements, each with a label and the number of hosts needed.
Click "Calculate Subnets" to run the VLSM algorithm.
The tool sorts requirements largest-first to minimize wasted space.
Each subnet gets the smallest prefix that satisfies the host count (accounting for network and broadcast addresses).
Results show the network address, mask, broadcast, first/last usable host, and remaining address space.

Use cases

Design enterprise network layouts with departments of different sizes.
Allocate cloud VPC subnets for dev, staging, and production environments.
Prepare CCNA and network engineering exam practice problems.
Split a /24 block into web, app, database, and management subnets.
Minimize address waste when allocating ISP-assigned blocks.
Document and verify existing VLSM network designs.
Plan point-to-point WAN links with /30 or /31 subnets alongside larger LAN blocks.
Audit IP address utilization efficiency across a multi-site organization.

Frequently Asked Questions

Last updated: 2026-06-10 · Reviewed by Nham Vu