Calculator input
This planner applies traditional IPv4 subnetting. It reserves network and broadcast addresses and supports buffered VLSM allocation.
Example data table
| Subnet Label | Requested Hosts | Buffered Hosts at 10% | Suggested CIDR | Total Addresses | Usable Addresses |
|---|---|---|---|---|---|
| Users | 120 | 132 | /24 | 256 | 254 |
| Servers | 60 | 66 | /25 | 128 | 126 |
| Voice | 30 | 33 | /26 | 64 | 62 |
| WiFi | 20 | 22 | /27 | 32 | 30 |
Formula used
- Buffered hosts: adjusted hosts = ceiling(requested hosts × (1 + growth buffer ÷ 100)).
- Required block size: block size = smallest power of two greater than or equal to adjusted hosts + 2.
- Subnet prefix: prefix = 32 − log2(block size).
- Total addresses: total = 2^(32 − prefix).
- Usable addresses: usable = total addresses − 2 for traditional IPv4 subnetting.
- Network range: broadcast = network + total addresses − 1.
- Utilization: utilization = allocated addresses ÷ base network total × 100.
The calculator can allocate in largest-first VLSM order, which usually reduces wasted address space and keeps larger host groups aligned cleanly.
How to use this calculator
- Enter the parent IPv4 network address and choose its prefix.
- Add the subnet labels and required host counts.
- Set a growth buffer if future expansion is expected.
- Keep largest-first allocation enabled for typical VLSM planning.
- Submit the form to generate the subnet table and graph.
- Review network, usable range, broadcast, and remaining pool details.
- Export the result as CSV or PDF for documentation.
FAQs
1. What does this calculator allocate?
It divides one IPv4 parent network into smaller subnet blocks based on host demand. It returns CIDR size, mask, usable range, broadcast, totals, and leftover space.
2. Why is largest-first allocation useful?
Largest-first VLSM usually reduces fragmentation. Bigger departments get aligned early, leaving smaller clean ranges afterward. That often improves fit inside the parent block.
3. Why are two addresses reserved?
Traditional IPv4 subnetting reserves one network address and one broadcast address in each subnet. The tool uses that model when it calculates usable host capacity.
4. Can I add a growth margin?
Yes. The growth buffer increases each host request before subnet sizing. This helps planners reserve extra capacity without manually recalculating every department.
5. What happens if the parent network is too small?
The calculator stops allocation and shows an error. You can reduce host requests, reduce growth buffer, or choose a larger parent network.
6. Does the tool normalize the base address?
Yes. If you enter a host address instead of the exact network boundary, the calculator normalizes it to the correct network address and displays a warning.
7. What does the graph show?
The graph compares allocated block sizes for each subnet and the remaining unallocated pool. It gives a quick visual view of address distribution.
8. Can I export the results?
Yes. The generated allocation table can be downloaded as CSV or exported as a PDF document for review, approvals, or deployment notes.