VLSM calculator
Divide an IPv4 network into subnets sized for your host requirements, with local VLSM allocation and Excel and CSV exports.
Input
JavaScript is required for local calculations. Documentation remains available.
Results
Enter the parent network and host requirements to calculate allocations.
With this allocation, remaining addresses form a contiguous range at the end of the parent network, but not necessarily a single CIDR subnet. Mathematical capacities: special-purpose ranges are not excluded.
| Name | Required hosts | CIDR network | Subnet mask | Wildcard | Network address | Broadcast | First host | Last host | Usable hosts | Total addresses |
|---|
Understanding VLSM
Complete example: 192.168.1.0/24
For 100, 50, 20 and 10 hosts: 192.168.1.0/25 (126 hosts, broadcast .127), 192.168.1.128/26 (62, broadcast .191), 192.168.1.192/27 (30, broadcast .223) and 192.168.1.224/28 (14, broadcast .239). Usable ranges are .1 to .126, .129 to .190, .193 to .222 and .225 to .238 respectively. 240 addresses are reserved, 16 remain, giving 93.75% utilization.
Frequently asked questions
What is the difference between VLSM and FLSM?
FLSM uses the same subnet mask length for every subnet. VLSM sizes each block for its own requirements: a LAN needing 100 hosts receives a /25, while one needing 20 receives a /27. Smaller LANs do not have to reserve the same large block as the largest LAN.
How is the smallest suitable subnet chosen?
The calculator finds the smallest block with enough LAN capacity after reserving a network address and a broadcast: /30 provides 2 hosts, /29 provides 6, /28 provides 14 and /24 provides 254. Even one host receives a /30. Neither /31 nor /32 is allocated; point-to-point allocation is not supported.
Why are the largest requirements allocated first?
Requirements are sorted by descending host count, then allocated from the start of the parent with CIDR alignment. Ties retain their input order. A /26 begins on a multiple of 64 addresses. This deterministic placement keeps every block within the parent without overlapping another block.
Why is a parent network rejected or too small?
The parent must be aligned: enter 192.168.1.0/24 rather than 192.168.1.10/24. Capacity must cover rounded block sizes, not just the requested hosts. An insufficient parent produces no partial allocation. Enter the parent in CIDR notation, not with a decimal mask. The form accepts 1 to 128 requirements, each from 1 to 4,294,967,294 whole hosts; existing subnets and exclusions are not managed.
What do remaining addresses and utilization mean?
Utilization compares reserved addresses, including network and broadcast addresses, with the parent block size. Here, the remainder is a contiguous range at the end of the parent, but it may require several CIDR blocks. It does not guarantee assignability or routability: special-purpose ranges are not excluded.
Should I export to Excel or CSV?
XLSX is selected by default and preserves eleven columns, text IP addresses and exact capacities without formulas. For UTF-8 CSV with a BOM, choose a comma or semicolon and select the same delimiter when importing. The initial FR/EN choice does not detect Excel settings. Formula-like names gain a protective apostrophe in CSV. Downloads are generated locally without network requests; the app keeps no input history. Reset clears results.