I read on a book about networks that class A has 16777214 hosts and 127 subnets,my question is: how they obtain the 127 number?

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    I'm voting to close this question as off-topic because is not specific to a *NIX environment, but instead network engineering. – Braiam Jun 17 '15 at 3:05

The A class networks space corresponds to all IP addresses with the first bit set to 0, i.e. IPs from to, i.e. the class A subnets from to

Since is reserved by the protocol (see RFC1122, section for details) you are left with the subnets from to

Anyway, classful routing is interesting only for historical reasons and is not relevant in any way to how modern network devices work.

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