IPv4 Subnet Split Calculator

Internet Protocol version 4 uses classless inter-domain routing prefixes to organise address space into routable networks. Splitting a parent block into smaller subnets improves security zoning, conserves addresses, and clarifies routing. Grasping how network, broadcast, and usable ranges change with each prefix protects against overlap and downtime.

The calculator lets you enter any host address, set the current prefix, and choose a deeper target prefix. A reactive interface validates inputs instantly, displays a summary card, and lists every resulting subnet in table and JSON views. You can copy or download the generated JSON to feed provisioning scripts or documentation.

Use this tool while planning virtual private clouds, segmenting on-premises LANs, or estimating host capacity before procurement. Caution: validate results against organisation policy before rollout.

Technical Details:

All calculations run entirely in your browser, using native bitwise operations for IPv4 arithmetic and a lightweight routing utility layer for presentation. No addresses, selections, or JSON exports leave your device, ensuring that sensitive topology data stays private while enabling instant, offline-capable computation.

Instant Validation

The interface checks address format and prefix logic on each keystroke, showing contextual error badges when values are invalid.

Multi-Prefix Support

You can convert any current prefix from /0 to /30 into a deeper target prefix, generating up to 1 048 576 child networks.

Dual Presentation Modes

Toggle between a striped results table for quick scans and a formatted JSON block for integration with infrastructure-as-code pipelines.

Copy & Download Actions

Single-click buttons copy JSON to the clipboard or trigger a client-side file download, streamlining team collaboration.

Client-Side Privacy

All logic, rendering, and export functions execute locally; no external APIs or telemetry are invoked, protecting confidential network plans.

Calculations & Scoring:

The tool converts the dotted-decimal host address to a 32-bit integer and derives subnet boundaries as follows:

Network   = IP_INT & NETMASK
Broadcast = Network | (~NETMASK)
Usable    = [Network + 1, Broadcast – 1]
Children  = 2(target-CIDR – current-CIDR)

Example: Splitting 192.168.0.0/24 into /26 yields 4 networks, each with 62 usable addresses. The calculator highlights network, broadcast, and usable ranges for every child block.

Data Privacy: calculations occur locally; no data is transmitted or stored.

Step-by-Step Guide:

Follow these steps to generate your subnet list.

1. Enter a dotted-decimal host address in the IP Address: field — for example 192.168.0.1.
2. Select the existing prefix from Current CIDR to match your allocated block.
3. Choose the desired deeper prefix in Target CIDR to define child subnet size.
4. Review highlighted errors; if any appear in the red alert box, adjust inputs until validation passes.
5. Inspect the summary card and choose either Table or JSON tab to view results.
6. Click Copy JSON to place data on the clipboard or Download JSON to save it locally.

FAQ:

Find quick answers to common questions.