Free, interactive networking tools from Practical Networking — CIDR & subnet calculators, subnetting practice, and TCP congestion-control visualizers.

CIDR IP .com

Interactive CIDR/subnet mask slider that visualizes how CIDR notation affects an IP address in binary and decimal.

SubnetIPv4.com

Randomized subnetting practice problems with auto-fill, auto-check, and a full Subnetting Mastery video series.