VLSM question

I did not understand this VLSM question from learncisco.net
is my explanation correct

Question: 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

Correct answer: D

Explanation :

10.0.0.0/8

/20 is the mask
10.0.0.0/20
block size 16
10th subnet is 10.0.9.0/20
further subnetting this with a /30 mask
the second pt-to-pt network would be 10.0.9.4/30 ...

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Comments

clarson

the networks support 254 hosts. that is a /24
so 10th network is 10.0.9.0/24
point to point networks are /30
block size is 4
second /30 network is 10.0.9.4/30

satishtech

Thank You.

randell09

Hi Guys

My Name is Randell and i am new to this. Can anyone please explain to me in more detail as i am confused right now. How does one get to the tenth subnet being 10.0.9.4/30?

I would appreciate your assistance.

randell09

my apologies . meant to say tenth subnet being 10.0.9.0/20
and the second pt-to-pt network 10.0.9.4/30 ?

clarson

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

randell09

Hi Clarson

Thank you for clearing that up. it makes more sense to me now. much appreciated