VLSM Calculator – Variable Length Subnet Mask Calculator
Variable-length subnet mask planning with automatic subnet allocation.
Introduction
Allocate multiple subnets with different sizes inside one parent network automatically.
How to use this vlsm calculator tool
- Enter parent network CIDR (e.g. 192.168.0.0/24).
- Add rows: subnet name and required hosts.
- Click Calculate VLSM.
- Review allocation table.
What Is This Tool?
VLSM (Variable Length Subnet Masking) assigns different prefix lengths to subnets so address space is not wasted.
Common in enterprise LAN design and CCNA advanced subnetting.
How to Use This Tool
- Enter parent network CIDR (e.g. 192.168.0.0/24).
- Add rows: subnet name and required hosts.
- Click Calculate VLSM.
- Review allocation table.
Formula / Calculation Logic
Sort requirements by size (largest first).
For each subnet, choose smallest prefix that fits hosts; align to next boundary; advance cursor.
Examples
Sample inputs and expected outputs:
| Input | Result |
|---|---|
| 192.168.0.0/24 — LAN 100, WAN 2, DMZ 10 | Three subnets: /25, /30, /28 style allocation |
Understanding Results
- Network — allocated subnet ID.
- Prefix — CIDR length chosen.
- Host range — usable addresses.
- Mask — dotted decimal mask.
Use Cases
- Campus network design
- CCNA VLSM labs
- Minimizing wasted IPs
Benefits
- Automatic alignment
- Sorted efficient allocation
- Table for documentation
Common Mistakes
Avoid these errors when using this network calculator:
Planning pitfalls
- Parent too small for all subnets
- Underestimating required hosts (forget servers + gateway)
- Overlapping subnets if done manually
References
Disclaimer
This calculator is for education, lab work, and network planning. Always verify production firewall, routing, and cloud VPC settings before deployment.
vlsm calculator — frequently asked questions
A VLSM calculator applies standard IPv4 subnet math (RFC 950 / CIDR) to compute network boundaries, masks, and host counts without manual binary conversion.
You enter IPv4 addresses, masks, or CIDR notation. The calculator bitwise-ANDs the address with the mask to find the network ID, then derives broadcast, wildcard, and host ranges.
Use it during CCNA study, VPC design, firewall rule documentation, IPAM planning, and troubleshooting when you need quick confirmation of subnet boundaries.
Yes. VSPIC runs calculations in your browser with no account required.
These calculators focus on IPv4. For IPv6 prefix planning, use our IPv6 Test and IP Subnet Calculator IPv6 tab.
CIDR writes the prefix length after a slash (e.g. /24). It replaces legacy classful networks and is used in routing tables worldwide.
A subnet mask marks which bits belong to the network portion. A /24 equals 255.255.255.0 with 254 usable hosts in typical subnets.
Yes. RFC 1918 addresses (10.x, 172.16–31.x, 192.168.x) use the same subnet mathematics as public space.
Set all host bits to 1 in the subnet — bitwise OR of network address with the inverted mask.
Variable Length Subnet Masking uses different prefix lengths within one parent network to minimize wasted addresses.
Largest host requirement first (standard VLSM practice) to reduce fragmentation.
Next step for vlsm calculator
Continue with ip lookup on VSPIC.
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