"The Great Exception" --CBTNuggets

Kingfisher · November 2008

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."

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

dynamik · November 2008

IMHO, these "short-cuts" are more confusing and convoluted than they're worth.

All I've done is memorize the values associated with 1-12 bits.

bits - networks (or hosts if you subtract 2)
1 - 2
2 - 4
3 - 8
4 - 16
5 - 32
6 - 64
7 - 128
8 - 256
9 - 512
10 - 1024
11 - 2048
12 - 4096

You don't even have to memorize all of them. The previous bit is always going to be half the value of the one you're on, and the next bit is going to be double the value. If you just remember that 5 is 32 and 10 is 1024, you can double or halve your way to any value you need. I don't see any reason why you need to mess around with any of those tricks. If you need 54 networks, well you'll obviously need six bits. The only change you have to make with hosts is to add two to whatever the required number of hosts is in order to accommodate the broadcast and network addresses. For example, if you were required to have 127 usable hosts, you'd have to add two and go with eight bits.

JavonR · November 2008

Kingfisher wrote:

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."

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

I honestly can't give you the technical explanation of why you need to follow the "great exception". However, you do NEED to follow it if you subnet the Jeremey way. Eventually (and this happened to me) you will come across a specific # of subnets that requires this and if you don't do it you will spend HOURS on the question trying to figure it out. Just follow the rule

genXrcist · November 2008

Hey there,

Funny, I just finished watching that vid too!

The thing is, the great exception ONLY applies if someone asks you for those specific number of Networks or Hosts. So if someone asks you for 2, 4, 8, 16, 32, 64 or 128 networks, then you -1 from either of those requests.

If they ask you for 1 host/networks, 3 hosts/network, 7 hosts/network, 15 hosts/networks etc., then you +1 to those requests.

In all other cases you follow his 3 standard steps.

I was with Jeremy as he explained this on the Network side but the Host side didnt make sense to me until you actually wrote it down in your reply so thanks!

MachX85 · June 2009

(EDIT) Never mind, I see what GenXrcist is saying. So you only have to do it on those specific numbers, but it would work if you did it for every number. right?

I just watched these videos and thought he tought subnetting very well.

when I got to the great exception though, I thought he was saying

subtract 1 if you when working with 0,1,2,3,4,5,6,7 finding network bits

and add 1 when working with 3,7,15,31,63,127 finding host bits

but then he lists examples where he applies it to 30 and 25, not the numbers he listed

so im not sure if I should use these rules for every number im subnetting or just the two groups he listed

WillTech105 · June 2009

Does it have something to do with the way he subnets that one would have to employ this great exception trick?

I get what Jeremy is aiming at but my main concern is for Cisco certs and even on the job would it be correct to do this.

sandman748 · June 2009

The subtract 1 rules seems to work for any given number of subnets. IE 8 networks - 1 = 7 (3bits), 7 networks - 1=6 (still 3 bits) etc...

The reason for the -1 is being able to use subnet 0 as a valid network.

The add 1 rule also seems to work. 32 hosts +1 = 33 (6 bits) , 31 hosts + 1 = 32 (still 6 bits), 30 hosts +1 (5 bits) etc...

The reason for the +1 is the network and broadcast addresses not being usable.

So as general rules, these seem to work for any given number of needed subnets or hosts.

"The Great Exception" --CBTNuggets

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