Subnet Calculator
Calculate subnet details and split networks.
Subnet planning is essential when designing office networks, cloud infrastructure, and container overlays. Utilitoo's Subnet Calculator accepts an IP address and prefix length, then returns network details including mask, broadcast, first and last host, and usable address count.
Optionally enter a larger prefix to split the network into smaller subnets - useful when dividing a /24 into multiple /26 segments for departmental VLANs or availability zones.
Select common prefix lengths from the dropdown or enter a custom mask. Results update with a clear breakdown and optional subnet split table. Pair with CIDR Calculator for notation-based input or DNS Lookup for hostname resolution.
All calculations run locally in your browser. Free and private for network engineers, sysadmins, and cloud architects.
Subnet Calculator expands an address and prefix into ranges and optional child subnets. It supports network design and study without binary worksheets. Keep a diagram of parent and child prefixes in the same ticket as the calculator screenshot so reviewers see intent, not only numbers. When cloud wizards suggest defaults, reconcile them with this breakdown before clicking apply in production accounts.
Common use cases
- Splitting a /24 into departmental /26 segments.
- Documenting usable hosts after reserving network and broadcast.
- Lab exercises for CCNA-style subnetting practice.
- Planning Azure or AWS subnet maps before Terraform apply.
- Explaining wildcard masks alongside CIDR Calculator output.
Tips & common mistakes
Start from the parent prefix you actually own. Leave headroom for growth. Pair with CIDR Calculator when input is slash notation rather than IP-plus-mask fields. Sketch the parent network on paper, then verify each child prefix does not overlap before applying firewall rules. Cloud consoles often reserve extra addresses - leave spare capacity. Document gateway and DNS IPs beside the calculator output for the team.
Common errors
- Overlapping child prefixes when splitting unevenly.
- Forgetting cloud providers reserve extra addresses beyond network/broadcast.
How to use
- Enter IP and prefix.
- Optionally enter a new prefix to split.
- Click Calculate.
Frequently Asked Questions
How do I calculate a subnet from an IP?▼
Enter the IP and prefix (e.g. /24), click Calculate, and view network, mask, and host range details.
Can I split a subnet?▼
Yes. Enter a new larger prefix to divide the network into smaller subnets with a breakdown table.
IPv4 or IPv6?▼
This tool supports IPv4 subnet calculations.
What prefix lengths are common?▼
/24, /25, /26, and /27 are common for office and cloud subnets. /30 is typical for point-to-point links.
Is data sent to a server?▼
No. Subnet calculations run entirely in your browser.
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