AWS Certified Advanced Networking – Specialty ANS-C00 – Question238

You have just provisioned a new VPC a with a CIDR block of 172.16.12.0/24. The entire CIDR block is fully utilized by subdividing it into 6 subnets, we will refer to these as Subnet1 through to Subnet6. The first 2 subnets (Subnet1 and Subnet2) are the same size. The last 4 subnets (Subnet3, Subnet4, Subnet5,
Subnet6) are also the same size. Subnet5 is half the size of Subnet2. The address space as occupied by the first two subnets is contiguous, as is the address space occupied by the last 4 subnets. Within Subnet3
AWS reserves the address 172.16.12.129 for the VPC router.
Select the correct IP address reserved by AWS for DNS in the Subnet2.

A. 172.16.64.1
B. 172.16.64.65
C. 172.16.12.66
D. 172.16.12.64

Correct Answer: C

Explanation:

Explanation:
From the documentation above we know AWS reserves the address x.x.x.1 for the VPC router, and x.x.x.2 for DNS from within each subnet. This question states that Subnet 3 reserves 172.16.12.130 for the VPC router. Given that we now know that the Subnet 3 (the 1st of the last 4 Subnets) starts at 172.16.12.128 – then it must follow that Subnet2 ends at 172.16.12.127. From here we know we have 128 addresses that are halved evenly between Subnet1 and Subnet2 – 128/2 = 64 or /26 in CIDR form.
Therefore it follows that the address reserved by AWS for DNS in the Subnet2 must be 172.16.12.66
Reference: http://docs.aws.amazon.com/AmazonVPC/latest/UserGuide/VPC_Subnets.html