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Posted by Feng on October 22, 2006, 2:13 pm
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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.
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Posted by gregg johnstone on October 22, 2006, 2:38 pm
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Feng wrote:
> 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.
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
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Posted by Drake on October 22, 2006, 6:34 pm
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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.
>
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.
--
Posted via a free Usenet account from http://www.teranews.com
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Posted by q_q_anonymous@yahoo.co.uk on October 23, 2006, 9:53 am
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Feng wrote:
> 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.
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!
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Posted by Aubrey Adams on October 25, 2006, 7:02 am
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>
> Feng wrote:
>> 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.
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.
> With classful you can't use the first and last.
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.
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