Sub net, 2^n or 2^n - 2?

Question

To calculate how many bits I need to borrow when designing a subnet, is it

2^n = X

or

2^n - 2 = X

X being the number of subnets desired.

I've found two websites each giving one of those equations.

On one of the sites it says that you can't use the "first and last subnetworks" hence the - 2.

I'm wondering if the author wasn't confusing with the first and last host addresses of a subnetwork, where the first one is used to identify the subnet and the last one is used for broadcasting.

Answer 1

The first formula is correct, you can indeed use the first and last subnet.

So for 8 subnets, you'd need 3 bits: 2^3