Hello all,

I am studying for the CompTIA Network+ exam and have a question about subnetting. The example given is that a cafe requires 3 different networks. One for guests, one for employees and another for wireless. The formula for determining how many subnets you create is 2^y. In this case, we want to create 3 therefore, 2^2 which is 4. So why is it that in a subnet of, you only go back two bits vice 4? I can't help but think that there is some common knowledge here that I am missing.

As always, any assistance is greatly appreciated.

Edited by Radio2006

4 Years
Discussion Span
Last Post by JorgeM

Ok, the traditional equation is 2^n-2. The reason is the not all routers will support the use of the first and last subnet IDs. There is an explanation if you are interested.

I have a high level overview if you are interested:

If they are teaching 2^n these days, that's fine. For three subnets, you would use two bits for four subnets.

I also have an online subnet calculator use can use to apply different masks and see the results.

The subnet mask you can use for three subnets using your formula is We moved to the right 2 bits.

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