The worst feeling in self-study is doing 20 subnetting problems and having no idea if you got them right. A good solutions PDF doesn't just give you the final IP address; it shows you the steps :

Apps are for engineers who already know subnetting. Videos are for conceptual understanding.

The best PDFs flip the script. Instead of "Given IP/Mask, find X," they ask:

Given the IP address 10.0.0.100 and a subnet mask of 255.255.255.192 , find:

: A comprehensive instructor-version workbook that includes binary charts, VLSM exercises, and step-by-step solutions. PacketLife Subnetting Cheat Sheet