Second guessing number of subnets

Guys,

I started going through the MeasureUp practice exams yesterday as part of the review process. I've just run across the fourth example where the explanations given for subnetting questions specifically calculate the number of subnets available as 2^n-2. I'd say it's a typo except not only do they repeat it over and over, but they also calculate it that way and state the answers that way!!

The latest example gives 172.16.97.100/20

In the explanation, they provide you a very clear explanation of why the number of subnets is 14!!

The first time I saw it, I thought it was a cut and paste error. The second time I was uncertain and used your subnet tool to back up my understanding. At this point I'm sitting here wondering if there is some alternative subnet understanding I skipped.

Am I missing something or is this just wrong?

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Comments

hodgey87

The 2^n-2 is used where ip subnet zero isnt in use. This means that you cant use the 1st usable address range and the final usable address range.

0.0 16.0 32.0 .......240.0
0.1 16.1 32.1 .......240.1
15.254 47.254 47.254.....255.254
15.255 47.255 47.255.....255.255

Please forgive me if that subnetting is wrong its been a long day

KPWright

AHA!!

Thanks for the end of a long day reminder. I do remember a very brief discussion of the ip subnet-zero command that degraded quickly into anecdotal ruminations of the "old days"

of Cisco switches. Anyway, I just went back and read the writeup on it and it does now make sense.

I also found enough posts in previous threads to come away with the understanding that the exams should allow you to assume ip subnet-zero is enabled unless the question specifically states otherwise. Is that your understanding as well?

Thanks again for the late night assist. Sleep well.

hodgey87

Yeh i believe that ip subnet zero is enabled by default these days anyways, i wouldnt worry to much about it.

When going for the exam the question will state whether whether it is enabled or not, just thoroughly read the question through before committing to an answer, ive been burnt many times not reading questions properly

billscott92787

KPWright wrote: »

Guys,

I started going through the MeasureUp practice exams yesterday as part of the review process. I've just run across the fourth example where the explanations given for subnetting questions specifically calculate the number of subnets available as 2^n-2. I'd say it's a typo except not only do they repeat it over and over, but they also calculate it that way and state the answers that way!!

The latest example gives 172.16.97.100/20

In the explanation, they provide you a very clear explanation of why the number of subnets is 14!!

The first time I saw it, I thought it was a cut and paste error. The second time I was uncertain and used your subnet tool to back up my understanding. At this point I'm sitting here wondering if there is some alternative subnet understanding I skipped.

Am I missing something or is this just wrong?

They are definitely showing you an example where ip subnet-zero is not in use. If it is in use you use 2^n = subnets (ip subnet-zero), if it is not in use (no ip subnet-zero) you use 2^n-2 = subnets. So,

The latest example gives 172.16.97.100/20

Which gives us a subnet mask of 255.255.240.0 So in the third octect we are going to have four network bits:

11111111.11111111.11110000.00000000

So, as you can see we have four network bits (1's). 2^4 = 16 - 2 = 14. If ip subnet-zero was in use you would have 16 (2^4) = 16.

In addition as you can see we have four network bits (1's). 2^4 = 16 - 2 = 14. There are 12 host bits (0) which gives us 2^12-2 = 4096 - 2 = 4094 hosts per subnet.

More of an example:

Since we have 16 subnets (2 reserved) we can only use 14 subnets. For example you would have the following subnets from this address:

172.16.0.0 /20 (reserved) Zero Subnet
172.16.16.0 /20
172.16.32.0 /20
172.16.48.0 /20
172.16.64.0 /20
172.16.80.0 /20
172.16.96.0 /20
172.16.112.0 /20
172.16.128.0 /20
172.16.144.0 /20
172.16.160.0 /20
172.16.176.0 /20
172.16.192.0 /20
172.16.208.0 /20
172.16.224.0 /20
172.16.240.0 /20 (reserved) "All Ones" Broadcast subnet

If it is not stated, that there is no ip subnet-zero then it (ip subnet-zero) is in use. It is enabled by default. If it is not, you will be told that it is not in use. (no ip subnet-zero)