Build a Subnet Plan
Use the fields below to analyze a subnet, estimate host fit, and generate a simple VLSM allocation plan.
Example Data Table
| Scenario | Base Address | Prefix | Required Hosts | Resulting Network | Usable Hosts |
|---|---|---|---|---|---|
| Office LAN | 192.168.10.34 | /24 | 50 | 192.168.10.0/24 | 254 |
| Voice VLAN | 10.10.22.80 | /27 | 20 | 10.10.22.64/27 | 30 |
| Branch Router Link | 172.16.5.2 | /30 | 2 | 172.16.5.0/30 | 2 |
| Wireless Segment | 192.168.50.129 | /25 | 100 | 192.168.50.128/25 | 126 |
Formula Used
A prefix of /n means the first n bits are network bits.
Mask = 32-bit value with n leading ones.
Network Address = IP Address AND Subnet Mask
Broadcast Address = Network Address OR Wildcard Mask
Total Addresses = 2^(32 - CIDR)
Usable Hosts = 2^(32 - CIDR) - 2
Typical exception: /31 is used for point-to-point links.
Find the smallest block where:
2^(host bits) - 2 ≥ required hosts
Sort host requirements from largest to smallest. Assign the smallest valid subnet to each segment in sequence.
How to Use This Calculator
- Enter a network name and an IPv4 address inside the subnet you want to inspect.
- Choose whether you want to provide the prefix as CIDR or as a dotted subnet mask.
- Enter the required host count to test whether the current subnet can support that demand.
- Set the gateway offset if you want a suggested gateway address inside the block.
- Add one host requirement per line to generate a simple VLSM design proposal.
- Click Calculate Subnet Design to display the results above the form.
- Use the CSV and PDF buttons to export the calculated results.
FAQs
1. What does CIDR mean in subnet design?
CIDR shows how many leading bits belong to the network portion. A larger prefix creates smaller subnet blocks with fewer usable host addresses.
2. Why does the calculator show network and broadcast addresses?
These define the boundaries of the subnet. The network address identifies the block, and the broadcast address marks the highest address in that subnet.
3. How is usable host capacity calculated?
Usable hosts usually equal total addresses minus two. One address is reserved for the network identifier, and one is reserved for broadcast.
4. What is the difference between subnet mask and wildcard mask?
A subnet mask marks network bits with ones. A wildcard mask is its inverse and is often used in routing and access list matching.
5. Why sort host needs from largest to smallest in VLSM?
This reduces wasted address space. Larger segments are harder to place later, so allocating them first makes the plan more efficient.
6. Can this tool help with gateway planning?
Yes. The gateway offset lets you suggest a likely gateway inside the subnet, such as the first usable address or another chosen position.
7. What happens if my host requirement does not fit?
The tool flags the mismatch and also suggests a prefix and subnet mask that can support the requested number of hosts.
8. Is this suitable for branch links and LAN planning?
Yes. It works for common LAN blocks, VLAN planning, point-to-point links, and early-stage subnet sizing before deployment.