Calculator Inputs
Enter an IPv4 address and prefix to analyze the current subnet. Optional planning fields also suggest better CIDR sizes for host and subnet targets.
Example Data Table
| Example IP | Prefix | Mask | Network | Broadcast | Usable Hosts |
|---|---|---|---|---|---|
| 192.168.1.44 | /24 | 255.255.255.0 | 192.168.1.0 | 192.168.1.255 | 254 |
| 192.168.1.44 | /26 | 255.255.255.192 | 192.168.1.0 | 192.168.1.63 | 62 |
| 10.20.30.40 | /27 | 255.255.255.224 | 10.20.30.32 | 10.20.30.63 | 30 |
| 172.16.5.200 | /28 | 255.255.255.240 | 172.16.5.192 | 172.16.5.207 | 14 |
| 203.0.113.10 | /30 | 255.255.255.252 | 203.0.113.8 | 203.0.113.11 | 2 |
Formula Used
Mask = first n bits set to 1, remaining bits set to 0, where n = CIDR prefix.
Network = IP Address AND Subnet Mask
Broadcast = Network Address OR Wildcard Mask
Wildcard = 255.255.255.255 − Subnet Mask
Total Addresses = 2(32 − CIDR)
For standard subnets, Usable Hosts = 2(32 − CIDR) − 2
Required pool = Required usable hosts + reserved hosts + 2
Host bits = ceiling(log2(Required pool))
Recommended prefix = 32 − Host bits
Borrowed bits = ceiling(log2(Required subnets))
Target prefix = Parent prefix + Borrowed bits
How to Use This Calculator
- Enter the IPv4 address you want to analyze.
- Enter the current CIDR prefix, such as 24 or 27.
- Optionally add a parent prefix for subnet planning.
- Enter required usable hosts to get a recommended subnet size.
- Enter required subnets to split a parent network evenly.
- Use reserved extra hosts when future growth must be included.
- Click Calculate Subnet to see results above the form.
- Download CSV or PDF files for documentation and reviews.
FAQs
1. What does the CIDR prefix mean?
The CIDR prefix shows how many bits belong to the network portion of the address. For example, /24 means 24 network bits and 8 host bits.
2. Why are two addresses often unavailable?
In standard IPv4 subnets, the first address is the network ID and the last address is the broadcast address. Those two values are not assigned to normal hosts.
3. What is a wildcard mask?
A wildcard mask is the inverse of the subnet mask. It is often used in routing, filtering, and access control configurations where matching flexible bit patterns matters.
4. What is the difference between public and private ranges?
Private ranges are reserved for internal networks and are not directly routable on the public internet. Public addresses are globally routable when assigned by an upstream provider.
5. Why does this tool show address class information?
Address classes are mostly historical, but they still help users recognize traditional network boundaries quickly. Modern subnetting usually relies on CIDR rather than classful addressing.
6. Can I use this calculator for VLAN planning?
Yes. It helps compare subnet sizes, usable hosts, and equal subnet splits, which makes it useful for VLAN planning, address allocation, and network segmentation work.
7. Why might the recommended host prefix be larger than expected?
The planning logic includes network and broadcast overhead for standard host subnets. It can also include extra reserved hosts when you want growth headroom built into the recommendation.
8. What do CSV and PDF exports include?
The exports include the main subnet summary and any generated subnet preview rows. They are useful for project handoffs, audits, client reports, and internal documentation.