subnet mask question

This is a question from cisco academy:

How many usable class C networks are created with a subnet mask of

The answer is 14. However, I think it is 16, because the binary of 240 is 11110000, so the usable networks are x.x.x.0, x.x.x.16, ..., x.x.x.240. That's 16 networks all together. Could anybody tell me what's wrong in my reasoning? Thanks.

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I understand that you dont use the subnet zero- and because you only go upto 240 that makes it 14


I am sure I will bw corrected-unless it states it is a classless routing protocol in use? Hope this helps

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gregg johnstone

You can't use a host with all 0's (network) or with all 1's (broadcast)

so in general, if you have n bits for the host, then you can have

(2^n -2) hosts. The same applies to the subnet bits.

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the keyword is usable. usable/available/... . (there may be other terms, but those are the ones i've seen.)

With classful you can't use the first and last. If they asked for "possible" as opposed to usable/available, then the answer would be 16.

Perhaps if anybody notices other terms for "possible", they could point them out and list them in this thread.

2^4 =16
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That is because to compute the number of subnets you use the formula 2 raiseto x, where x is the number of subnet bits. > This is a question from cisco academy:

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The whole "you can't use the first and last subnet" thing is a quaint ancient historic quirk of the current Networking Academy CCNA 1 course. Watch the use of the word of "useable" versus "total" or "possible". In this regard it also helps *not* to refer to the first, second, tenth subnet either. Use the terms Subnet 0, Subnet 1, Subnet 9, etc.

Nothing to do with classful/classless routing - the IOS command to enable the use of the first and last created IPv4 subnets is "ip subnet-zero" which is set on by default in all router IOS's these days.

Reply to
Aubrey Adams

ok, that was your response to the OP

By the way, it's not a historic quirk in CCNA. It's a historic fact. Keyword being history. But that's how it got into the CCNA.

these days classful addresses aren't given out.

But the CCNA exam still talks of classful, and when it does, it's talking HISTORY.

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Yes, it's a fact for sure, but these days in my view it's quirky fact that can unnecessarily confuse students (and some instructors). And I'm pretty sure the CCNA cert exam no longers requires candidates to distinguish between usuable and non-useable subnets in design questions.

Classful boundaries are still used by default in many areas such as EIGRP and when statically configuring Windows hosts. Given classful addressing is still applied I would suggest "legacy" might be a better term, unlike the subnet issue above which is obsolete and hence historic.

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Aubrey Adams

out of interest, and i've wondered about this for a long time.

Why were classful boundaries reserved? I recall that RFCs , specifically RFC 3330, says no more than "reserved for classful reasons"

I see, that when "network number" or better, "net number" , that's usually understood as excluding class identification bits - is defined as all 0s or all 1s, then we get the classful boundaries. But my question is what are the reasons for those being reserved?

my thoughts in response to the question are fairly scattered. Who is(or better, was) identifying all networks in a Class. Or broadcasting to all networks in a class? Did they think somebody would? Or is it just/only done in keeping with convention and custom to reserve all 0s and all 1s, because at the time *they thought* it's usually wise - e.g. i think they even once said that the all 1s subnet should be reserved. Potty really, especially reserving a whole subnet just to say all subnets all hosts. But that's just poor memory. My main point is before the e.g. , regarding the classful boundaries

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240 means 4 bits for the subnet and 4 for hosts. You might find it as /28 4 bits will give you 16 combinations, but the first is used for subnet ID and the last is used for broadcast ID. Therefore, there will be only 14 IDs for hosts. You will find it as a famous formula (which is great to confuse people) 2*n-2, where n is the number of borrowed bits.

eager and beaver

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