I’ve noticed that many networking students have few problems answering questions about the simpler, classful networks and their subnet masks. But then many struggle when problems involve CIDR networks.
I think it’s because beginners have grasped the basics using some rules of thumb, but not the underlying principles.
When we take the class C netmask, 255.255.255.0, the shortcut thought is that the magic number 255 means “part of network”, and magic 0 means “part of host address”. But if that’s all you know, then as soon as you see a netmask like 255.255.192.0, you’re stumped. What does the 192 mean?
The two keys to decrypting 172 are understanding what a mask, generically, is, and thinking in binary, instead of base-10.