Subnet Calculator

Exports use the latest calculated or generated results.

Formula Used

How to Use This Calculator

1. Enter an IPv4 address and CIDR prefix.
2. Choose a calculation mode when planning capacity.
3. Use the splitter for equal-size child networks.
4. Use VLSM when subnet host demands differ.
5. Open the IPv6 tab for prefix calculations.
6. Download results as CSV or PDF when finished.

Subnetting Guide

Understanding Network Boundaries

Subnetting divides a larger IP network into smaller logical networks. Each subnet receives a network address and address range. IPv4 networks also normally reserve a broadcast address. The CIDR prefix determines how many bits identify the network. A larger prefix creates smaller networks with fewer host addresses. A smaller prefix creates larger networks with more addresses. Subnet masks express the same boundary using dotted decimal notation. For example, a /24 prefix equals 255.255.255.0. The remaining eight bits represent host addresses inside that network.

Planning Hosts and Subnets

Network planners usually begin with expected host counts and growth. Traditional IPv4 subnets reserve the first and last addresses. Therefore, host calculations often add two addresses before sizing. Point-to-point /31 networks are an important modern exception. They can use both available addresses on supported equipment. A /32 identifies one exact IPv4 address. Equal-size subnetting works well when departments need similar capacity. VLSM is better when different groups need different subnet sizes. VLSM assigns larger blocks first to prevent fragmentation. This tool sorts host requirements automatically before allocating address blocks.

IPv6 and Practical Network Design

IPv6 removes the broadcast address concept used by IPv4. Prefix lengths still separate network and interface portions. A /64 remains common for standard IPv6 LAN segments. IPv6 address counts can become extremely large. This calculator uses large integer arithmetic for accurate prefix ranges. It also identifies common address categories and special ranges. Binary displays can help students understand how masks create boundaries. Export functions make subnet plans easier to document and review. Always verify production designs against router and firewall requirements. Leave capacity for growth when allocating long-lived network ranges. Document gateway addresses before deployment to prevent configuration mistakes. Consistent naming also simplifies troubleshooting and future audits. Good subnet planning keeps networks efficient secure and scalable.

Common IPv4 CIDR Reference

Frequently Asked Questions