Everything you need to master CIDR, VLSM, and modern cloud subnetting — short lessons, an interactive quiz, a complete /0 to /32 reference, a glossary, and a printable cheat sheet. Free, no signup.
The Learn Center walks you from raw binary to comfortable CIDR fluency. The interactive lessons cover IP address structure (the 32-bit layout, octet boundaries, why dotted-decimal is just a presentation format), subnet masks and how the network/host split works, the magic-number method that lets you do subnet math without converting to binary, CIDR notation as a compact replacement for classful addressing, network and broadcast address derivation via bitwise AND and OR, and Variable-Length Subnet Masking for efficient address allocation across heterogeneous networks.
Beyond the fundamentals, the lessons cover topics that bite engineers in production: how AWS, Azure, GCP, and OCI each reserve different numbers of IP addresses per subnet, why /28 is the smallest practical AWS subnet, when /31 is appropriate per RFC 3021, how to plan Kubernetes pod CIDRs so they do not overlap your VPC, and the difference between subnet masks and the wildcard masks Cisco ACLs use. The streak-tracked quiz reinforces each concept with practice problems; the printable cheat sheet gives you a one-page reference for the desk.
Everything in this Learn Center is free and requires no signup. Your lesson progress and quiz streaks are stored in your browser's local storage — never sent to a server. If you prefer to learn by reading complete articles instead of stepping through lessons, the blog has long-form pieces on VLSM design, AWS VPC reserved IPs, IPv6 addressing, and Kubernetes networking. To verify what you learn against real calculations, the subnet calculator, VLSM planner, and cloud-aware tool are one click away.
A complete one-page PDF with the /0–/32 reference table, cloud reserved-IP rules, and binary subnetting shortcuts. Pin it to the wall.
⤓ Download Cheat Sheet PDFMost engineers grasp the core math in 2-4 hours of focused practice. The interactive lessons here cover binary notation, mask derivation, network and broadcast calculation, VLSM, and supernetting. The quiz reinforces it with streak-tracked practice problems.
Memorizing /24, /28, and /30 is enough for most day-to-day work — those are the prefixes you'll reach for. For certifications, the cheat sheet on this site is a printable reference that covers every prefix and the magic-number method for fast calculation.
Classful (the original IPv4 design) divided addresses into fixed Class A (/8), B (/16), and C (/24) blocks. Classless (CIDR, post-1993) allows arbitrary prefix lengths — /20, /27, /30 — which makes address allocation far more efficient. Modern networks are all classless; classful terminology survives as a casual shorthand.