VLSM Calculator

Design efficient networks with Variable Length Subnet Masking. Create subnets of different sizes based on actual host requirements to minimize wasted IP addresses.

254
Subnet Requirements

Add the subnets you need with their required number of hosts. The calculator will find the optimal subnet size for each.

Hosts needed
Hosts needed
Hosts needed

What is VLSM?

VLSM (Variable Length Subnet Masking) is a subnetting technique that allows network administrators to divide an IP address space into subnets of different sizes. Unlike traditional fixed-length subnetting where all subnets must be the same size, VLSM enables efficient allocation by matching subnet sizes to actual requirements.

Before VLSM, if you needed three subnets with 100, 50, and 10 hosts respectively, you'd have to use a mask that accommodates 100 hosts for all three – wasting significant address space in the smaller subnets. VLSM allows each subnet to have just enough addresses for its needs, plus room for the network and broadcast addresses. For basic subnet calculations, see our subnet calculator.

VLSM Advantage

A network needing 100, 50, and 10 hosts with fixed subnetting would require three /25 subnets (378 addresses total). With VLSM, you use /25 + /26 + /28 = only 208 addresses, saving 170 addresses for future use.

How to Use the VLSM Calculator

Step 1: Define Your Major Network

Enter the network address and prefix length of the address block you want to subnet. This is typically the range assigned by your ISP or allocated from your organization's larger pool. For example, if you've been assigned 192.168.1.0/24, enter those values.

Step 2: Add Subnet Requirements

For each subnet you need, enter a descriptive name and the number of hosts required. Be realistic about requirements – include all devices that need IP addresses (computers, servers, printers, phones, network equipment). Add a reasonable buffer (20-50%) for growth.

Step 3: Calculate Allocation

Click "Calculate VLSM Allocation" to see the optimal subnet assignment. The calculator automatically sorts requirements by size (largest first) and assigns the smallest subnet that meets each requirement. This greedy algorithm produces efficient allocations.

Step 4: Review Results

The results show each subnet's network address, mask, usable range, and utilization. The visual bar displays how address space is distributed. Review the efficiency score – higher is better, indicating less wasted space.

VLSM Allocation Algorithm

The calculator uses a standard VLSM approach:

  1. Sort by size: Requirements are sorted from largest to smallest. This ensures large subnets get contiguous address blocks.
  2. Calculate subnet size: For each requirement, find the smallest prefix that provides enough usable hosts (hosts + 2 for network and broadcast).
  3. Allocate sequentially: Starting from the network address, assign each subnet in order, advancing the next available address after each allocation.
  4. Track remaining space: Keep count of unallocated addresses for future expansion.
VLSM Example

Network: 192.168.1.0/24 (254 usable hosts)

Requirements: 100 hosts, 50 hosts, 20 hosts, 10 hosts

Allocation:

  • 100 hosts → /25 (126 usable) → 192.168.1.0/25
  • 50 hosts → /26 (62 usable) → 192.168.1.128/26
  • 20 hosts → /27 (30 usable) → 192.168.1.192/27
  • 10 hosts → /28 (14 usable) → 192.168.1.224/28

Remaining: 192.168.1.240/28 (14 hosts available for future use)

Understanding Efficiency

The efficiency score measures how well address space is utilized:

  • 90%+ (Excellent): Minimal waste. Address space is well matched to requirements.
  • 70-89% (Good): Reasonable allocation with some room for growth.
  • 50-69% (Fair): Significant unused space. Consider if requirements are accurate or if a smaller major network would suffice.
  • Below 50% (Poor): Large amounts of wasted space. Review requirements or use a smaller address block.

Some "waste" is intentional – leaving room for growth is good planning. The goal isn't 100% utilization but rather appropriate sizing that balances current needs with future expansion.

VLSM Best Practices

Plan for Growth

Don't size subnets exactly to current needs. Add 25-50% buffer for growth. It's easier to have unused addresses within a subnet than to renumber later when you run out.

Reserve Space

Keep some address space unallocated for future requirements. The calculator shows remaining addresses after allocation – ensure this meets your future needs.

Document Everything

Record your VLSM allocation with subnet purposes, creation dates, and responsible teams. Good documentation prevents conflicts and simplifies troubleshooting.

Consider Aggregation

When possible, allocate contiguous addresses to related subnets. This allows route summarization, reducing routing table sizes. For example, keep all department subnets within the same /22 block.

VLSM vs Fixed-Length Subnetting

Aspect Fixed-Length (FLSM) Variable-Length (VLSM)
Subnet sizesAll same sizeDifferent sizes allowed
Address efficiencyOften poorTypically excellent
Planning complexitySimpleModerate
Router requirementsClassful routing OKRequires classless routing
Best forUniform networksVaried requirements

For equal-sized subnet division, our visual subnet calculator provides a graphical approach. For simple CIDR calculations without VLSM, use the CIDR calculator.

Protocol Support

VLSM requires routing protocols that support classless routing (carry subnet mask information in routing updates):

  • Supported: OSPF, EIGRP, IS-IS, BGP, RIPv2
  • Not supported: RIPv1, IGRP (legacy protocols)

All modern networks use classless routing, making VLSM universally applicable in current deployments.

Hosts per Prefix
PrefixUsable Hosts
/24254
/25126
/2662
/2730
/2814
/296
/302

Frequently Asked Questions

Sorting largest-first ensures optimal address alignment. Large subnets have stricter alignment requirements (a /25 must start at .0 or .128), while small subnets are more flexible. Allocating large subnets first guarantees they get properly aligned addresses, then smaller subnets fill the remaining space efficiently.

The calculator will show an error if total requirements exceed available hosts. You'll need to either use a larger major network (smaller prefix like /23 instead of /24), reduce your host requirements, or split requirements across multiple network blocks. Remember that each subnet needs additional addresses for network and broadcast, so usable hosts are always less than total addresses.

Include all devices needing IP addresses: routers (usually 1 per subnet for the gateway), switches with management IPs, wireless access points, printers, IP phones, and user devices. A typical formula is: end-user devices + servers + network equipment + 25% growth buffer. Don't forget that routers connected to each subnet need an IP in that subnet.

The calculator assigns addresses optimally based on the sorting algorithm. While you could manually reassign addresses, the calculated order (largest first) is mathematically optimal for minimizing fragmentation. If you need specific subnets at specific addresses for organizational reasons, note the requirements and manually adjust the configuration while ensuring proper alignment.

The calculator supports subnets from /30 (2 usable hosts, for point-to-point links) to whatever your major network allows. A /24 major network can have subnets up to /24 (same size, no division). Practically, /30 is the minimum useful subnet as it provides 2 hosts; /31 is special (point-to-point only, no broadcast) and /32 is a single host route.

Related Network Tools

Looking to calculate subnets for your network? You can also perform IP-based subnet calculations. For detailed analysis, choose masks for different network sizes. Network administrators often need to plan networks using visual tools. Additionally, see all addresses in CIDR blocks. Many users find it helpful to design IPv6 address schemes. For comprehensive planning, find ending addresses for subnet ranges.