Subnetting Question

mzinz

I'm taking a practice quiz, and came upon this question:

You are presented with an IP address with a prefix of /22. How many more subnets can you add to your design if you further subnet with a VLSM mask of /27?

A: 256
B: 64
C: 32
16

But wouldn't it matter what IP/class is being used in the example??
If we were using class A, and had /27, then we would have far more subnets available than if we were using class C.

Am I not understanding?

BTW: It said the correct answer was C, which is just 2^5. 27-22 = 5.

georgemc

mzinz,
NO, it wouldn't matter what the class of the IP address is. They presented you with a given mask of /22, which is not the standard class A, B, or C mask. Therefore you know that we're using VLSM and ignoring class boundaries.

With a /27 mask, you take 5 of your 10 available host bits and use them for subnetting.

That gives you 2^5=32 subnets with 5 (10-5=5) bits leftover for hosts, for a total of 2^5-2 usable hosts in each subnet.