need a confirmation on subnetting

[vishalprashar]

<p>Can anyone please confirm it to me that it is right to say that, when calculating a subnet mask, the point where we borrow bits from the host portion in order to form new subnets we use the formula <strong>2 to the power of n - 2 </strong>for classful subnetting while we use the formula <strong>2 to the power of n in all other cases.</strong></p>

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[Giuseppe Larosa]

Hello Vishal,

the old rule was 2^n -2 for skipping the all zero subnet (no ip subnet-zero) and for avoiding to have the last subnet broadcast to looks like the same as the broadcast of the original major network.

The new rule that takes advantage of classless routing protocols that  carry the subnet mask is instead  2^N and the first and the last subnets are now usable and not reserved

Hope to help

Giuseppe