Subnet Calculator

Understand IPv4 and IPv6 subnetting and why masks, prefixes and block splitting matter so much in network planning.

Subnetting can look intimidating at first because it mixes binary logic, capacity planning and address organization, but the core idea becomes much clearer once you understand what the prefix controls. This section explains CIDR, broadcast, usable range, VLSM and IPv6 delegation, connecting theory with the practical questions that appear in real network planning.

How to use the result without losing context.

1. Enter the main value and confirm that its format matches the expected pattern for IPv4 and IPv6 subnets.
2. Read the result together with supporting details, especially network, broadcast, mask, prefix, usable hosts, VLSM and IPv6 delegation.
3. For professional use, record the date, source or reference context and any assumptions before making an operational decision.

Use this tool on your site

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Sources and references for this tool

These references help contextualize formulas, standards, APIs and limitations used on this page. They do not replace professional validation when a result has legal, financial, medical or operational impact.

• RFC 791, Internet ProtocolIETF, Historical basis for IPv4 and addressing used in subnetting.
• RFC 4632, Classless Inter-domain RoutingIETF, Reference for IPv4 CIDR.
• RFC 4291, IPv6 Addressing ArchitectureIETF, IPv6 addressing model.
• RFC 3021, Using 31-Bit Prefixes on IPv4 Point-to-Point LinksIETF, Special-case /31 use on IPv4 point-to-point links.

Frequently asked questions

Where to go next