'Total networks' calculation

Hi, I'm currently reading thought the CCENT book but Im a bit confused over some information on chapter 12.
Heres the info for class A B C networks:

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Can anyone explain to me why the 'Total networks' is using powers of 7? What happens to the extra bit?

Thanks!

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WastedHat

Correction: powers of 2 with multiples of 7

stryder144

This requires the binary conversion process, which many people do not like. 2 to the 8th power would be 256. In that case, all of eight places in that octet would be able to be turned on (11111111, though we must remember that we start with 0 and end with 255, which equates to 256 separate values). For class A networks, the format is 00000000 through 01111111. Thus, only seven out of the eight places can be either a 0 or a 1. The first one cannot be switched on at all. That is why they say 2 to the 7th power.

Class A = 00000000 - 01111111
Class B = 10000000 - 10111111
Class C = 11000000 - 11011111

clarson

the 127.x.x.x network is reserved for loopback addresses.

WastedHat

Ahh it's starting to make more sense to me, thanks! Is it correct to say that because they pre-defined the ranges of the first octet into class, the full range isnt used for any one class meaning the combinations of network addresses in A B C are reduced to 2^7, 2^14, 2^21?

clarson

yes.
class A first bit is always zero. so, usable bits is 8 - 1 which is 7
class B first 2 bits are always 10 so usable bits is 16 - 2 which is 14
class C first 3 bits are always 110 so usable bits is 24 - 3 which is 21

WastedHat

Thank you very much!