VLSM Calculator
Allocate differently sized subnets.
VLSM Calculator: inputs and results
Enter an IP address, CIDR prefix, or range. Then check its first and last address below.
Calculated Boundaries & ParametersReady
Key Capabilities
- •100% Client-Side: Zero telemetry or server latency.
- •RFC Compliant: Exact 32-bit and 128-bit integer precision.
- •Quick Copy: Instant access to CIDR, mask, and range bounds.
- •Cross-Platform: Works on desktop, tablet, and mobile browsers.
Related Planning, VLSM & CloudAll Tools
VLSM Allocation Validator
Check alignment, capacity, and overlaps.
Supernet Calculator
Find a covering parent prefix.
Exact CIDR Aggregator
Merge prefixes without adding addresses.
Route Summary Calculator
Summarize networks and identify included gaps.
DHCP Scope Calculator
Plan allocation boundaries within a subnet.
DHCP Exclusion Calculator
Remove specified addresses and ranges.
How VLSM Calculator Works?
This tool works by sorting subnet requests from the largest host count to the smallest, sizing each one to the smallest prefix that fits, and placing each subnet at the next aligned address in the parent block.
Enter the parent block
Type the address space to divide, such as
192.168.10.0/24.List the host requirements
Enter each subnet with its host count: Sales 100, Engineering 50, HR 20, and onward.
Size and sort
Each count maps to a prefix (100 hosts → /25). Larger subnets are placed first.
Allocate in order
Each subnet starts at the next free address aligned to its size. The output lists network, range, broadcast, and spare hosts.
VLSM Calculator Formula
The formula is Prefix_i = 32 − ceil(log2(Hosts_i + 2)), with each subnet placed at the next aligned start after the previous subnet.
Prefix per request
p_i = 32 − ceil(log2(H_i + 2))Smallest subnet that fits H_i hosts.
Aligned start
S_i = ceil(Cursor ÷ 2^(32−p_i)) × 2^(32−p_i)Keeps each subnet on its boundary.
Next cursor
Cursor = S_i + 2^(32−p_i)Moves past the placed subnet.
Space used
Σ 2^(32 − p_i)Must not exceed the parent size.
Worked example: 192.168.10.0/24
Sales 100 hosts → 192.168.10.0/25 Engineering 50 hosts → 192.168.10.128/26 HR 20 hosts → 192.168.10.192/27 Management 10 hosts → 192.168.10.224/28 WAN link 1 2 hosts → 192.168.10.240/30 WAN link 2 2 hosts → 192.168.10.244/30 Used 248 of 256 addresses
VLSM Calculator Conversion Table
The table shows the full VLSM plan for 6 subnets inside 192.168.10.0/24, sorted from largest to smallest.
| Subnet | Hosts Needed | Prefix | Network | Usable Range | Broadcast | Spare Hosts |
|---|---|---|---|---|---|---|
| Sales | 100 | /25 | 192.168.10.0 | 192.168.10.1 – 192.168.10.126 | 192.168.10.127 | 26 |
| Engineering | 50 | /26 | 192.168.10.128 | 192.168.10.129 – 192.168.10.190 | 192.168.10.191 | 12 |
| HR | 20 | /27 | 192.168.10.192 | 192.168.10.193 – 192.168.10.222 | 192.168.10.223 | 10 |
| Management | 10 | /28 | 192.168.10.224 | 192.168.10.225 – 192.168.10.238 | 192.168.10.239 | 4 |
| WAN link 1 | 2 | /30 | 192.168.10.240 | 192.168.10.241 – 192.168.10.242 | 192.168.10.243 | 0 |
| WAN link 2 | 2 | /30 | 192.168.10.244 | 192.168.10.245 – 192.168.10.246 | 192.168.10.247 | 0 |
What is VLSM Calculator?
The VLSM Calculator is a tool that divides one IPv4 block into subnets of different sizes, each sized to its own host requirement.
Variable Length Subnet Masking (VLSM) uses a different mask for each subnet. Fixed-length subnetting gives every subnet the same size, so a 2-host WAN link wastes as much space as a 100-host LAN. VLSM gives the WAN link a /30 and the LAN a /25 from the same parent.
Placing the largest subnet first prevents alignment gaps. Smaller subnets then fill the space after it without breaking any boundary.
What are the uses of VLSM Calculator?
There are 5 main uses of the VLSM Calculator, including branch design, address conservation, and exam preparation.
Designing branch networks
A single /24 covers user, voice, printer, management, and WAN subnets at a small site.
Conserving address space
Each subnet takes only the space it needs, which leaves larger free blocks for growth.
Preparing for CCNA labs
VLSM design questions appear in every CCNA exam version.
Planning cloud subnet tiers
Web, app, and database tiers of different sizes share 1 VPC block.
Documenting address plans
The output table becomes the subnet register for a site.