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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
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The A class networks space corresponds to all IP addresses with the first bit set to 0, i.e. IPs from 0.0.0.0 to 127.255.255.255, i.e. the class A subnets from 0.0.0.0/8 to 127.0.0.0/8.

Since 0.0.0.0/8 is reserved by the protocol (see RFC1122, section 3.2.1.3 for details) you are left with the subnets from 1.0.0.0/8 to 127.0.0.0/8.

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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