Hands-on practice

• subnettingpractice.com — endless generated subnetting problems
• messwithdns.net — a real DNS playground (Julia Evans)
• badssl.com — every kind of TLS failure, safely
• test-ipv6.com — check your IPv6
• waveform.com/tools/bufferbloat — bufferbloat grading
• Cisco Packet Tracer (free with registration) or GNS3 — build virtual networks
• TryHackMe / HackTheBox — legal hands-on security labs