This isn't a Cisco specific question, but I have seen a recent subnetting thread here so thought it would be more valid in this forum than Misc.

I am studying towards 70-291 (what a beast!) and have come accross a question in the MS book that I don't really understand. I am starting to read the excellent guide JeremyW post here, and am starting to view the tutorials and Learn to Subnet but was wondering if someone would mind explaining the question I am stuck on, so I have something to work on;

You are going to subnet the network 199.205.15.0/24 using VLSM. Your routers support the zero subnet and the longest match algorithm. Someone has suggested some network/subnet masks. Which are not valid?;

199.205.15.64/25

199.205.15.192/26

199.205.15.130/26

199.205.15.96/26

199.205.15.8/27

199.205.15.40/28

The MS Press book states;

Correct Answers: A, C, and E

A- Correct: Mask = 255.255.255.128

Increment = 256-128 = 128

Because 128 is not a factor of 64 (64 is not divisible by 12, this network is not valid.

B- Incorrect: Mask = 255.255.255.192

Increment = 256-128 = 64

64 is a factor of 192, so this network is valid.

C- Correct: The Value in the fourth octet must be a power of 2, for example, 128, 64, 32 and so on. This network is not valid.

D- Incorrect: Mask = 255.255.255.224

Increment = 256-224 = 32

32 is a factor of 96, so this network is valid.

E- Correct: Mask = 255.255.255.240

Increment = 256-240 = 16

16 is not a factor of 8, so this network is not valid.

F- Incorrect: Mask = 255.255.255.248

Increment = 256 - 248 = 8

8 is a factor of 40, so this network is valid.

I am sure they are correct, but the explanations are not *enough* due to my lack of understanding.

Thanks for reading.

I am studying towards 70-291 (what a beast!) and have come accross a question in the MS book that I don't really understand. I am starting to read the excellent guide JeremyW post here, and am starting to view the tutorials and Learn to Subnet but was wondering if someone would mind explaining the question I am stuck on, so I have something to work on;

You are going to subnet the network 199.205.15.0/24 using VLSM. Your routers support the zero subnet and the longest match algorithm. Someone has suggested some network/subnet masks. Which are not valid?;

199.205.15.64/25

199.205.15.192/26

199.205.15.130/26

199.205.15.96/26

199.205.15.8/27

199.205.15.40/28

The MS Press book states;

Correct Answers: A, C, and E

A- Correct: Mask = 255.255.255.128

Increment = 256-128 = 128

Because 128 is not a factor of 64 (64 is not divisible by 12, this network is not valid.

B- Incorrect: Mask = 255.255.255.192

Increment = 256-128 = 64

64 is a factor of 192, so this network is valid.

C- Correct: The Value in the fourth octet must be a power of 2, for example, 128, 64, 32 and so on. This network is not valid.

D- Incorrect: Mask = 255.255.255.224

Increment = 256-224 = 32

32 is a factor of 96, so this network is valid.

E- Correct: Mask = 255.255.255.240

Increment = 256-240 = 16

16 is not a factor of 8, so this network is not valid.

F- Incorrect: Mask = 255.255.255.248

Increment = 256 - 248 = 8

8 is a factor of 40, so this network is valid.

I am sure they are correct, but the explanations are not *enough* due to my lack of understanding.

Thanks for reading.

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