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# CISCO CCENT subnetting confusion

Posted on 2010-09-21

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