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CISCO CCENT subnetting confusion

Posted on 2010-09-21
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Last Modified: 2012-05-10
I have been doing some Cisco training,
"The Great Exception" --CBTNuggets first part of the CCENT training

I was going well with subnetting learning from CBTNuggets videos with the practice problems and I was getting all of them correct. However, now that I come to the "Great Exception" section, I find myself lost. They are basically stating:

"When subnetting based on the number of networks, SUBTRACT 1 from the number.
When subnetting based on the number of hosts per network, ADD 1 to the number."

I mean its a simple rule to just blindly follow, but the explanation is hard to digest and frankly, the accompanying document for this is not clear as well.

For example, in the document it states that "If I were to break a network into 8 subnets, you would assume it takes four bits since 8 in binary is (00001000). However you can achieve this requirement with only three bits since 0-7 is really 8 numbers (0,1,2,3,4,5,6,7). If you work out the problem by reserving only three bits, you will get exactly eight subnets." I dont know why I cant understand this. as all the other courses had been normal, then comes this different way of doing it. not sure how or where it fits.

The problem is that I solved this problem using 3 bits like they state and I do get 8 subnets. However, in previous problems, I did not follow this rule (subtract 1 for networks) and I got correct answers (according to the video). How is using 4 bits for 8 networks now wrong and now I need to use 3 bits. And if so, how come this wasn't taught at the very beginning of the section. Or, do I need to follow this if the exam asks for specific numbers.

If anyone can help me get this through my head I would appreciate it. It really has done my head in. I sat and did all S/N Classes and then this on the end quickly taught with no relevence or reference to where it fits. or I have been infront of the machine toooooo long and brain has stalled. so anyone who can help explain it in stupid man terms, much appreciated.

sorry for such a silly question i am sure.

Ancients
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Assisted Solution

by:prvnkumark
prvnkumark earned 50 total points
ID: 33724139
Hi ,

You  will correct answer with 3 and 4 bits , But here jermy asks to use 3 bits cos, if use 4 it will be more then what you want. And in host case it will be shortage if you dont add.

To calculate network.. 2power of x , where x = number of subnet bit.
I.e  2 to power of 4= 32
2 to power of 3=16

For ip address formula is (2 to power x)- 2
I.E  ( 2 to power of 4)-2=32-2 =30.

I hope you understood. If could not explain you correctly . I will try again.

Regards,
Praveen
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by:raxix
raxix earned 50 total points
ID: 33724440
In normal subnetting we don't use the 0 subnet but now a days cisco routers support 0 subnet so you don't have to -1 if your router supports subnet 0.
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by:Don Johnston
ID: 33724598
In the past, all zeros and all ones subnets were discouraged (RFC 950).

http://www.cisco.com/en/US/tech/tk648/tk361/technologies_tech_note09186a0080093f18.shtml

However, many people didn't worry about the all ones subnet (as would appear to be the case with your book).

On Cisco routers, it was possible to use the all zeros subnet but you had to issue the command "ip subnet-zero". Starting with version 12.0, this feature is now on by default.

Bottom line is that today (and for CCNA certification), subnet zero and the all ones subnet are allowed.
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by:Ancients
ID: 33760781
So this isnt something used when breaking the IP to work out the subnet. it applies to all? It was a CBT Nugget training, that covered subnets in one 54 minutes, and one 30 give or take minutes, all of which make no mention of this when working out hosts, or networks for various IP models. with heaps of emphasis on learning what was taught, with many senario examples given. then on the third video we do the reverse engineering the process from existing network IP's.  and then are told we have to add one to even numbers, and minus ones to odds.

I understand the calculating number of hosts/networks with the x being the power of 2 (-2) and 0 for the first bit.

I suppose why be taught to do a certain type of subnetting, then after all that effort and hours of playing with it to be quick and able to work it out faster without going through the whole process, then add the minus plus when working out the number of bits,

to me 500= 00000101 which equals 3 in the initial models given. are their just certain numbers this applies too. just not sure if it applies to he whole or part of the subnetting lesson, or if it also applies to the breaking of IP rather then figuring out the IP range between networks.

Thanks for answers so far posted.  and sorry for confusion. as this was a confusing lesson. found many google replies to the same course which was great except for this area. as we are told do it this way, then suddenly do it that way.
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Don Johnston earned 400 total points
ID: 33760866
I don't know how they teach it or what methodology they use.

All I know is that for the exam, the number of subnets is all available (zeros and ones).
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by:Ancients
ID: 33763366
so then no need to worry about deleting ones and adding ones to odds or evens in either case of networks or hosts subnetting or when breaking?

Thanks all.

DJ your link really helped clear it up for me

thanks :)

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Author Closing Comment

by:Ancients
ID: 33763375
Thanks for your efforts, I shared in the responce as my question may have lead to other answers and I can see they had been answered also.

DJ your reply and link helped me go over my notes and understand it better. so thanks all.

AJ
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Expert Comment

by:Don Johnston
ID: 33763902
>so then no need to worry about deleting ones and adding ones to odds or  evens in either case of networks or hosts subnetting or when breaking?

No. For the subnets there is no need to delete anything. For the hosts, you always have to delete two.
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by:Ancients
ID: 33774036
Thanks DJ :)
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