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VLSM subnet planner

Allocate named IPv4 subnets by host requirements and see the remaining CIDRs.

◉ Stays in your browser

01 / Your address plan

Parent IPv4 CIDRAn unallocated address block with prefix /0–/30. Host bits are normalized and reported; decimal octets cannot have leading zeros.Subnet requirements: Label,HostsOne per line, up to 128 requirements and 32,768 characters. Labels are 1–80 characters with no commas. Hosts must be positive whole numbers, without separators or leading zeros. Blank lines are ignored.

Allocates the largest host requirement first, with input order breaking ties. Each subnet reserves its network and directed broadcast addresses; even one requested host gets at least a /30. Include gateways, infrastructure, and planned growth in your host counts.

This planner starts with the whole parent available. It does not import existing allocations or implement /31 point-to-point links and /32 host routes.

Calculated in your browser. Your inputs are not uploaded and no addresses are contacted.

02 / Your result

Your result will appear here.
Start with your own input or try an example.

What it does

Build an IPv4 address plan for departments, VLANs, or a networking exercise from a parent block and named host requirements. The planner chooses the smallest conventional subnet for each requirement, allocates the largest requests first, and shows network addresses, masks, host ranges, broadcasts, and spare capacity. Unallocated space is returned as exact CIDRs for the next stage of your plan.

Useful when

Sketch subnet sizes for office, guest, camera, and management VLANs.
Check whether a parent address block can accommodate a set of host counts.
Learn largest-first variable-length subnet allocation with visible address ranges.

How to use it

Enter an available parent IPv4 CIDR with a prefix from /0 to /30.
List up to 128 named requirements as Label,Hosts, including gateways and any planned growth.
Generate the complete plan, inspect allocations and unallocated CIDRs, then copy or download it.

A few useful details

How does the planner choose subnet sizes and order?

Each requested host count gets the smallest power-of-two address block that also reserves two addresses for the network and directed broadcast. Requests are allocated by descending host count; equal requests retain input order. Blocks begin at aligned boundaries inside the parent, and the full plan is rejected if any request does not fit.

Why does one or two hosts receive a /30 rather than a /31?

This tool plans conventional multi-access subnets with two reserved addresses, so /30 is its minimum. RFC 3021 permits /31 on supported point-to-point links, which are outside this planner’s scope. /32 host routes are also excluded. Include router interfaces and other infrastructure in your requested usable-host count.

Does the plan account for existing assignments or provider reservations?

No. The parent starts entirely available, and arithmetic alone does not verify ownership, device availability, special-use ranges, or cloud-provider reservations. Host-bit inputs are normalized with a notice. The tool accepts up to 128 requirements and 32,768 characters, preserves labels, and runs locally without scanning a network.