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I have just started learning from the ICDN1 official Cert book.

I've just covered the concept of Classful Networks.

However when introducing Subnetting, the example has me confused.

It examples an internetwork using 'Five Subnets of Class B network 150.9.0.0', the subnets being 150.9.1.X, 150.9.2.X, etc.

However I'm confused: from what I understood of classful networks, class C IP addresses are defined by the first 3 octets, but to me it seems these are defined by the first 3 octets in a sense, also. I understand that subnetting is trying to sub-divide these networks even more, but how then do you differentiate between a class B subnet of x.x.x , and a class C network?

Obviously I am very new to networking, so apologies for the silly beginner question. I know the answer is something simple, and I'm confused about the concept, but I'm not seeing it.

marked as duplicate by Ron Maupin Oct 5 '18 at 13:29

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  • the first 3 bits.. not octets. Note that classfull addressing is deprecated for decades. It's a shame they still teach it. – JFL Oct 5 '18 at 12:35
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Here's the most important thing to know about classfull addressing:

It is obsolete, and has been since before you were born. The book you are reading is out of date.

Classful addressing has been replaced by Classless InterDomain Routing (CIDR).

You will not see classfull routing anywhere in real networks or on the Internet. The only place you will find it is in old books.

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    This is a shame to hear, because it is in the Official Cisco (Academic Ed.) 2017 ICDN1 study book. So outdated as it is, it seems for the exam at least I need to understand it... How odd – user2974706 Oct 5 '18 at 12:40
  • They ought to stop teaching those obsolete classes. They died in 1993 with RFC 1519. – Zac67 Oct 5 '18 at 12:52

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