VLSM Subnet Calculator
Allocate subnets by host count using VLSM
Enter a source network and host counts to allocate.
About this tool
VLSM Subnet Calculator divides a base network into variable-length subnets based on the host count each one needs, the way network engineers and CCNA students design real networks. Enter a source CIDR and a list of required host counts, and the tool sorts them largest-first, picks the smallest prefix that fits each, and lays out every block: prefix, network address, broadcast, range, and usable hosts. Allocating largest subnets first minimizes fragmentation and wasted addresses, which is the core of VLSM. All calculation runs locally in your browser β no input is uploaded.
Frequently Asked Questions
What is VLSM?
Variable-Length Subnet Masking. Instead of one fixed prefix for every subnet, you give each subnet the smallest prefix that fits its host count, which wastes far fewer addresses than fixed-length subnetting.
How does it decide the prefix for each subnet?
For a requested host count N, it finds the smallest prefix whose usable-host capacity (2^(hostBits) - 2) is at least N. For example 50 hosts needs a /26 (62 usable).
Why sort largest-first?
Allocating the biggest subnets first keeps the remaining address space contiguous and minimizes fragmentation, the standard VLSM best practice. Smaller blocks fit into the gaps left by larger ones.
What if my network is too small for all subnets?
The tool reports that the requested blocks exceed the source network capacity, shows how far it got, and tells you the total addresses needed versus available.
Does it include network and broadcast addresses?
Each block shows the network address, broadcast address, and usable host range separately, so you can see exactly which IPs are assignable.
Is my input uploaded?
No. All VLSM calculation happens locally in your browser. Your CIDR and host counts never leave your device.
Can it handle IPv6?
This tool focuses on IPv4 VLSM, the most common case for subnet design by host count. For IPv6 subnetting use the IPv6 Subnet Calculator.
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