Free Subnetting Tools
Master Subnetting — Free Interactive Practice
From CIDR basics to VLSM design, NetForge has every tool you need to ace the subnetting questions on your CompTIA Network+ exam.
What Is Subnetting?
Subnetting divides a large IP network into smaller, more manageable segments called subnets. Each subnet is a broadcast domain — devices can communicate directly within it, but need a router to reach other subnets. Subnetting is essential for network security, efficient IP address usage, and is heavily tested on the CompTIA Network+ (N10-009) exam.
Free Subnetting Tools
Everything you need, no account required
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IP Subnet Calculator
Enter any IP and CIDR prefix — instantly see network address, broadcast, host range, and wildcard mask.
FREE
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VLSM Planner
Allocate subnets of varying sizes to multiple departments using Variable Length Subnet Masking.
FREE
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IPv6 Address Basics
Interactive guide to IPv6 notation, prefix lengths, and address types with live examples.
FREE
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Subnetting Quick Sheet
Printable reference card with all subnet masks, wildcard masks, and host counts from /8 to /30.
FREE
Key Subnetting Formulas
Everything the exam tests
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Number of Hosts
Usable hosts = 2^(32 − prefix) − 2. The −2 removes the network address and broadcast address. A /24 gives 2^8 − 2 = 254 hosts.
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Number of Subnets
Subnets borrowed from host bits = 2^n, where n = bits borrowed. Borrowing 3 bits from a /24 gives 2^3 = 8 subnets, each a /27.
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Subnet Increment
The block size (increment between subnets) = 256 − subnet mask octet. For /26 (mask 192), block = 256 − 192 = 64. Subnets start at 0, 64, 128, 192.