subnet mask question

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This is a question from cisco academy:

How many usable class C networks are created with a subnet mask of
255.255.255.240?

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.


Re: subnet mask question

Feng wrote:
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I understand that you dont use the subnet zero- and because you only go
upto 240 that makes it 14
16,32,48,64,80,96,112,128,144,160,176,192,208,224,240

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


Re: subnet mask question
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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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Re: subnet mask question
Feng,

That is because to compute the number of subnets you use the formula 2
raiseto x, where x is the number of subnet bits.  In this case, 4. So 2
raiseto 4 is 16 minus 2 is 14.
Why subtract 2, that is because we are reserving the subnet where all
bits are 0 and all bits are 1. If am not mistaken, the all 0s,
represents the entire subnet, and the all 1s is the broadcast network
address.

Feng wrote:
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Re: subnet mask question

Feng wrote:
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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 <--possible
(2^4)-2=14 <--usable/available


There was an issue, It has been a while. A rule for networks and a rule
for hosts. I think it was that in classless, networks are 2^n , but
hosts are (2^n)-2. Whereas classful, both are (2^n)-2.
You chose an interesting one, classful, and taking 4 bits leaves 4
bits. So, had they asked how many usable hosts, as oppose to usable
networks, it'd have been the same answer. 14.
So you could even misread the question and get it right.

You overlooked the importance of the term usable, and just parroted it
when you answered about usable networks!


Re: subnet mask question

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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.

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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.



Re: subnet mask question

Aubrey Adams wrote:
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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.

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these days classful addresses aren't given out.

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


Re: subnet mask question

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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.


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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.



Re: subnet mask question

Aubrey Adams wrote:
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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


Re: subnet mask question

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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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