More VLSM with 10.0.0.0 /8

[John Cheetley]

Hi again Cisco techs,

Further to my previous question,

Got this question wrong.

You are implementing a VLSM network. You are using the Class A network of 10.0.0.0/8 and need to create networks that support 254 hosts that will be used for each LAN. The tenth network that was created will be used for addressing all the point-to-point links between routers at HQ, all the point-to-point WAN links, and any point-to-point links in the branch offices. 

What is the subnet ID of the second point-to-point network created?
A. 10.0.9.8/30
B. 10.0.9.2/30
C. 10.0.9.6./30
D. 10.0.9.4/30
E. 10.0.10.4/30
F. 10.0.10.2/30

My question is since when does a class a address mix with VLSM when VLSM is classless unless this is like a trick question?

I found a link from 2 years ago below which explained a little more..

http://www.techexams.net/forums/ccna-ccent/123053-vlsm-question.html 

Now the end of the reasoning shows more but jumps from the /24 to a /30 as below shows.... this is what confused me..

first of all the network your starting out with is 10.0.0.0 /8
each network needs to support 254 hosts. so you create /24 subnets for each network
10.0.0.0 /24 is first network
10.0.1.0 /24 is second
10.0.2.0 /24 is thrid
....
10.0.8.0 /24 is the ninth
10.0.9.0 /24 is the 10th /24 subnet or you start counting from zero. So tenth network is 10 - 1 or 9

the 10.0.9.0 /24 network is going to used for pt to pt networks use a /30 and their block size is 4
10.0.9.0 /30 is the first pt to pt second is 0 + the block size (or 4)
10.0.9.4 /30 is the second pt to pt

Thanks again :) 

[Georg Pauwen]

Hello,

your reasoning sounds perfectly sound. I also would chose:

D. 10.0.9.4/30

"My question is since when does a class a address mix with VLSM when VLSM is classless unless this is like a trick question?"

Not sure if I understand your question. You don't mix 10.0.0.0/8, you simply cut it up into smaller networks. As long as those don't overlap, there is no problem.

[Kekun · Accepted solution]

I think you understood it correctly.

He just needs to cut the 10.0.9.0 /24 network into smaller /30 subnets to create all the ptp links

Inside this 10.0.9.0 /24 network.

And yes, D is the correct answer :)

[John Cheetley]

Hi Georg, 

thanks for the reply. Since we are on this topic, can you please explain what the difference is between these addresses below.

Which of the following options are examples of VLSM? (Choose three)
A. Taking the 192.168.1.0/27 and subnetting it to multiple /30s
B. Taking 192.168.1.0/24 and subnetting it to multiple /29s
C. Taking 172.16.1.0/24 and subnetting it to multiple /26s
D. Taking 10.1.24.128/26 and subnetting it to multiple /28s
E. Taking 172.16.0.0/16 and subnetting it to multiple /24s

Are these suppose to be broken down to determine this?

[mateuschapman]

I believe the answer is 10.0.9.6, since a /30 onlu uses 2 host bits, which adds up to 4 addresses and 2 useable addresses.
Therefore the 2nd /30 would start at 10.0.9.5 and end at 10.0.9.8 and the useable addresses would be .6 and .7

I could be wrong but thats my reasoning anyway

[Joseph W. Doherty]

No, because the 2 usable addresses (2nd and 3rd) are within the 4 addresses for a /30.

1st /30 of a /24 block .0..3

2nd /30 of a /24 block .4..7

BTW, /31 might be used for p2p, then 2nd subnet would be .2/31.