Free, interactive networking tools from Practical Networking.
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.
TCP Congestion Control Visualizer
Interactive TCP Reno and Cubic congestion window visualizer with slow start, fast recovery, and congestion avoidance phases.