Need help with Class B VLSM

[sabregod]

Hello to all,

I've been practicing subnetting for hosts requirement and I can't seem to figure it out. I've uploaded the question with instructor answers and my answer..

I'm at a dead end since i can't seem to know how he got those numbers. Am I doing something wrong? Here's my calculation:

https://i.imgur.com/D9DuZYz.jpg

Also, I don't know how the 3 reserved addresses he gave out relevant to this question at all. I'm a complete beginner so any help would be greatly appreciated.

[Julio E. Moisa]

Yes you can, because when you are executing VLSM you will begin from the highest requirement to the lowest, you will begin first with 350 and finishing with 2.

Remember the teacher used the subnet: 172.128.51.128/25 for 100 hosts (3rd requirement) so the network 172.128.51.0/25 is available to be subnetting and meet the following requirements 60, 20, 10, 5 ,2.

Imagine your teacher select the subnet 172.128.51.0/25 instead 172.128.51.128/25 we cannot use the network 172.128.51.76/30  because it is inside of 172.128.51.0. But it is not the case because your teacher used the other one.

:-)

[sabregod]

I think I remember my instructor telling me there's only one answer in solving these type of questions. Thanks anyway

[Julio E. Moisa]

Hi

Once I made the subnetting I used just 2 subnet from /20

172.128.50.0/X
172.128.51.0/X

There are not overlapping with 

172.128.56.0/21,
172.128.51.76/30
172.128.52.0/30 

Because they are different subnets, for example the first one:

172.128.56.0/21 it goes from:

172.128.56.0 to 172.128.64.255  (there are 8 subnets included on /21)

/21 = 255.255.248.0

Formula:

256 - 248 = 8

About the 172.128.52.0/30  that subnet was not involved on the subnetting because we only used 50 and 51.

:-)

[Julio E. Moisa]

Basically the teacher is saying dont use any subnet into the range 172.128.56.0/21, dont use 172.128.51.76/30 (we didnt use that) and 172.128.52.0/30 (we never used the subnet 172.28.52.X/X just 172.128.50.X/X and 172.128.51.X/X so we are good)

Please check the attachment with the complete subnetting.

:-)

[chrihussey]

So you weren't supposed to use 172.128.51.76 /30. In your answer you defined the 100 host subnet as 172.128.51.0 /25 which includes the 51.76 /30.

What your instructor did was assigned the 100 host subnet to the second half of the block (172.128.51.128 /25) and then assigned the remaining lower number subnets to the first half and worked around the 172.128.51.76 /30.

Hope this clarifies things.