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.

How do I do VLSM subnetting by hand?

List the subnets from most hosts to fewest. For each, find the smallest power of 2 that is at least the host count plus 2: 50 hosts need 64 addresses, a /26. Give it the next free address that is a multiple of its size, then move on. In 192.168.10.0/24 the 50-host /26 takes .0 to .63, so the 30-host /27 starts at .64.

Why does a 2-host WAN link get a /30 and not a /31?

The tool always keeps a network and a broadcast address, so 1 or 2 hosts get a /30 with 2 usable addresses. RFC 3021 allows /31 on point-to-point links and many routers support it, but this calculator does not hand out /31 or /32 blocks. If your equipment supports /31, you can split a /30 from the result into two /31 links.

VLSM (Variable Length Subnet Mask) Calculator

Enter a base network and a list of required host counts to get VLSM subnets sized to fit each host count, allocated largest first, with the leftover space listed.

VLSM (variable length subnet masking) splits one network into subnets of different sizes.

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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Quick answer

VLSM (variable length subnet masking) splits one network into subnets of different sizes. Sort the host counts from largest to smallest, give each the smallest block that holds its hosts plus 2 (network and broadcast), and allocate the blocks in order. For 192.168.10.0/24 with 50, 30, 14 and 2 hosts you get a /26, /27, /28 and /30, using 116 of 256 addresses.

Examples

Hosts needed	Subnet	Usable
50	192.168.10.0/26	62
30	192.168.10.64/27	30
14	192.168.10.96/28	14
2	192.168.10.112/30	2
Left over	.116 to .255	140 addresses

Enter a base network, then click Calculate Subnets.

Summary

This VLSM calculator allocates multiple subnets of different sizes from a single base network block. You specify how many hosts each subnet needs, and the tool assigns the smallest possible subnet prefix to each requirement — sorted largest first to avoid fragmentation. It shows the network address, subnet mask, broadcast address, usable host range, and CIDR notation for every subnet, plus how many addresses remain unused.

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 as /30 subnets alongside larger LAN blocks.
Audit IP address utilization efficiency across a multi-site organization.

Frequently Asked Questions

Last updated: 2026-06-13 · Reviewed by Nham Vu

VLSM (Variable Length Subnet Mask) Calculator

VLSM Configuration

Allocation Summary

Unallocated Address Space

Quick answer

Examples

Summary

How it works

Use cases

Frequently Asked Questions

VLSM (Variable Length Subnet Mask) Calculator

Use the VLSM (Variable Length Subnet Mask) Calculator

VLSM Configuration

Allocation Summary

Unallocated Address Space

Quick answer

Examples

Summary

How it works

Use cases

Frequently Asked Questions