Calculator Inputs
Use a parent IPv4 network, then choose how you want the child subnet plan generated.
Example Data Table
Example: 192.168.10.0/24 broken into /27 child subnets.
| # | Network | First Host | Last Host | Broadcast | Usable Hosts |
|---|---|---|---|---|---|
| 1 | 192.168.10.0/27 | 192.168.10.1 | 192.168.10.30 | 192.168.10.31 | 30 |
| 2 | 192.168.10.32/27 | 192.168.10.33 | 192.168.10.62 | 192.168.10.63 | 30 |
| 3 | 192.168.10.64/27 | 192.168.10.65 | 192.168.10.94 | 192.168.10.95 | 30 |
| 4 | 192.168.10.96/27 | 192.168.10.97 | 192.168.10.126 | 192.168.10.127 | 30 |
Formula Used
- Parent block size: 2(32 − parent prefix)
- Child block size: 2(32 − child prefix)
- Created subnets: 2(child prefix − parent prefix)
- Usable hosts per child: child addresses − 2, except /31 and /32 special cases
- Network address: floor(IP ÷ block size) × block size
- Broadcast address: network address + block size − 1
- First usable host: network + 1, except /31 and /32 special cases
- Last usable host: broadcast − 1, except /31 and /32 special cases
The calculator also derives the dotted subnet mask, wildcard mask, borrowed bits, and address step size from the chosen child prefix.
How to Use This Calculator
- Enter any IPv4 address that belongs to the parent network.
- Enter the parent CIDR prefix for that network.
- Select a planning method: child prefix, subnet count, or hosts per subnet.
- Fill in the active planning field for your chosen method.
- Set how many subnet rows you want displayed.
- Click the calculation button to generate the normalized parent network and child subnet table.
- Review the summary values, then export the visible output as CSV or PDF.
FAQs
1) What does subnet breakdown mean?
Subnet breakdown means dividing one larger IPv4 network into smaller child networks. Each child subnet gets its own network address, usable host range, and broadcast address for cleaner planning.
2) Why does the calculator normalize my entered IP?
The entered IP may be a host inside the parent network, not the network address itself. The calculator converts it to the proper parent network before generating any child subnet rows.
3) What are borrowed bits?
Borrowed bits are host bits converted into network bits. Borrowing more bits creates more child subnets, but each child subnet then has fewer usable host addresses.
4) Why are usable hosts usually two fewer than total addresses?
Traditional IPv4 subnetting reserves one address for the network identifier and one for broadcast. That leaves total addresses minus two for normal host assignment inside each subnet.
5) What is the wildcard mask used for?
A wildcard mask is the inverse of the subnet mask. It is commonly used in ACLs, routing policy statements, and network matching tasks where host bits must be expressed directly.
6) Which planning mode should I choose?
Use child prefix when you already know the final subnet size. Use subnet count when you know how many child networks are needed. Use hosts per subnet when device capacity matters most.
7) Does this calculator support IPv6?
No. This version is built for IPv4 subnet breakdown only. IPv6 addressing uses different notation, larger address space, and different design habits than classic IPv4 subnetting.
8) Why might I see only part of the subnet list?
Large parent networks can create many child subnets. The display limit prevents a huge page. Increase the row limit when you want a longer visible breakdown table.