Subnet Planning Form
Large screens use three columns, smaller screens use two, and mobile uses one.
Example Data Table
| Site | Segment | Parent | Required Subnets | Adjusted Hosts | Suggested Child | Sample VLAN Span |
|---|---|---|---|---|---|---|
| HQ West | User Access | 10.40.0.0/23 | 6 | 90 | /25 | 200 - 205 |
| DC One | Server Rack | 172.16.20.0/24 | 4 | 30 | /27 | 300 - 303 |
| Branch A | Voice | 192.168.50.0/24 | 8 | 14 | /28 | 400 - 407 |
Formula Used
Growth-adjusted hosts: ceil(current hosts × (1 + growth %)) + reserved hosts
Child subnet size: choose the smallest prefix where usable host count can hold the adjusted hosts.
Usable hosts: 2^(32 - prefix) - 2 for regular IPv4 subnets. For /31, two addresses are usable. For /32, one address is available.
Maximum child subnets: 2^(child prefix - parent prefix)
Network address: IP AND subnet mask
Broadcast address: network + total addresses - 1
Gateway suggestion: first usable address plus the chosen gateway offset minus one.
How to Use This Calculator
- Enter the site name and a segment label for the subnet group.
- Provide the parent IPv4 network and parent prefix.
- Set how many child subnets you need.
- Enter today’s host count per subnet.
- Add expected growth and reserved addresses for future use.
- Choose the gateway offset and starting VLAN.
- Submit the form to generate subnet ranges, gateway suggestions, and capacity metrics.
- Use the CSV or PDF buttons to export the generated plan.
Frequently Asked Questions
1) What does this IP planning tool calculate?
It sizes child subnets from your parent network, applies growth and reserved address buffers, suggests a subnet prefix, estimates VLAN ranges, and lists detailed network, usable, gateway, and broadcast values.
2) Why is the usable host count smaller than total addresses?
Standard IPv4 subnetting reserves the network and broadcast addresses, so usable hosts are usually two fewer than total addresses. The tool also treats /31 and /32 with their common special handling.
3) What happens when the requested plan does not fit?
The tool warns that the entered parent range is too small, shows how many child subnets can fit, and suggests the minimum parent prefix needed to support the requested design.
4) How is the gateway address chosen?
The gateway is calculated from the first usable address plus your offset. An offset of 1 picks the first usable address, while 2 picks the second usable address.
5) Can I use this for VLAN planning too?
Yes. The form accepts a starting VLAN, then increments it across the planned subnets. This helps align IP ranges and VLAN numbering during rollout planning.
6) Why did my entered network address change after submission?
If you entered a host IP with a parent prefix, the tool normalizes it to the actual network address. That ensures subnet math is based on the correct boundary.
7) What do the CSV and PDF exports include?
They export the calculated summary values and the generated subnet table. This makes it easier to share planning output with engineers, operations teams, or implementation partners.
8) Is this tool meant for IPv4 only?
Yes. This version is built for IPv4 subnet planning. The calculations, masks, broadcast logic, and range handling in this file are designed specifically for IPv4 addressing.