Grokking the Mask – understanding classless subnetwork masks

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,, 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, 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.

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