IPv4 subnetting divides a network into smaller address blocks so you allocate host capacity efficiently, isolate traffic, and simplify routing. Each subnet has a unique prefix length that determines the number of usable host addresses and broadcast boundaries. Understanding how prefixes translate into ranges is vital for reliable address management and avoiding overlapping allocations.
The visual subnet calculator accepts a parent CIDR and renders an interactive bar stack that mirrors the address space. Click a coloured block to split it into equal-size children, or click a parent to merge contiguous blocks. Below the graphic, a sortable table lists every subnet, its first and last address, and the exact count of usable hosts in real time.
Use this designer while planning data-center ranges, lab networks, or cloud VPCs, then copy the resulting CIDRs directly into firewall, routing, or DHCP configurations. The on-screen visualisation highlights wasted space so you iterate quickly without spreadsheets. Always validate proposed changes against current allocations and change-control policies before applying them in production.
Technical Details:
Everything runs client-side for speed and privacy.
This tool runs entirely in your browser using a lightweight reactive interface bound to a canvas-based bar component. Subnet objects are stored in an in-memory tree, ensuring every split and join updates both the graph and the summary table instantly without page reloads or server requests.
Computation functions translate IPv4 prefix lengths into dotted-decimal ranges and host counts with 32-bit bitwise arithmetic. User input is sanitised locally and no network packet leaves your device, safeguarding sensitive topology information. The design is responsive and respects prefers-color-scheme for accessible light or dark themes.
Real-time Visualisation
The color-coded bar stack resizes and animates with every split or merge, giving an immediate spatial understanding of free and allocated address space across the entire CIDR.
Interactive Split & Join
Click any block to cut it into equal halves, or select adjacent siblings to recombine, enabling flexible planning without memorising subnet maths.
Range Table Sync
A responsive table lists subnet address, first and last host, and usable count, updating instantly to match the visual graph for precise copy-and-paste output.
Fast Reset
Use the Reset button to clear all modifications and return to the original parent network state, encouraging quick what-if experiments without refreshing the page.
Client-Side Privacy
All calculations execute locally; no CIDR or host information is transmitted, stored, or logged, allowing safe use with confidential network designs.
Calculations & Scoring:
The calculator derives each subnet’s usable host count and address span from its prefix length.
Formula
usable hosts = 2(32 - mask) - reserved, where reserved equals 2 for masks < /31 and 0 otherwise.
Example
An address 10.0.0.0/26 yields 26 - 2 = 62 usable hosts spanning 10.0.0.1 to 10.0.0.62; /31 retains two usable point-to-point addresses.
Interpretation
Smaller prefixes increase host capacity but reduce available subnet blocks; larger prefixes offer tighter blast-radius control at the cost of address overhead.
Data Privacy: All computations stay in the browser; nothing is uploaded.
Step-by-Step Guide:
Follow these steps to design your subnet layout:
- Enter the parent block in the CIDR: input (e.g.
10.0.0.0/24). - Press Enter; the bar stack and table populate instantly.
- Click a coloured block to split it into two equal child subnets.
- Select a larger grey block to join contiguous children back into their parent.
- Review each row in Subnet Address and Hosts to confirm capacity.
- Click Reset to clear changes and start a new plan.
FAQ:
Find quick answers to common questions.
What is a CIDR?
A Classless Inter-Domain Routing notation combines an IP address with a prefix length to represent a network’s size and position within the IPv4 space.
How do I split subnets evenly?
Click any coloured segment; the calculator halves the prefix length, creating two equal child blocks displayed in both the bar stack and table.
Is my data stored?
No. All operations run locally in your browser. No inputs, outputs, or telemetry leave your device.
Why do usable hosts differ?
Traditional subnets reserve the network and broadcast identifiers; hence -2. Point-to-point masks (/31-/32) need no reservation.
Can I export the list?
Select the table, copy it to your clipboard, or right-click to save it as CSV in most browsers for quick import into documentation.
Troubleshooting:
Resolve common issues quickly.
| Issue | Solution |
|---|---|
| Blank display after CIDR entry | Verify correct syntax (a.b.c.d/len); masks must be 0-32. |
| Cannot join blocks | Only adjacent siblings with matching masks merge; ensure both are selected. |
| Reset button unresponsive | Check browser console for script errors; reload the page if necessary. |
| Table scrolls off-screen | Use the horizontal scrollbar or widen your browser window for better visibility. |
| Colour contrast hard to see | Switch your OS or browser to dark mode; the tool respects system themes. |
Advanced Tips:
Speed up complex designs with these expert hints.
- Plan from the widest prefix downward to visualise long-term growth before splitting smaller segments.
- Use /31 point-to-point links to conserve addresses on router back-to-back connections.
- Colour patterns reveal fragmentation; aim for contiguous green areas to simplify summarisation routes.
- Copy entire rows to spreadsheets; the order column preserves hierarchy for pivot-table diagrams.
- Save browser bookmarks with query-string presets (
?cidr=) to reopen designs instantly.