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Exam ANS-C00 topic 1 question 238 discussion

Exam question from Amazon's ANS-C00
Question #: 238
Topic #: 1
[All ANS-C00 Questions]

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
Show Suggested Answer Hide Answer
Suggested Answer: C 🗳️
From the documentation above ג€" 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
by Souslik1999 at Nov. 22, 2020, 8:10 a.m.
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eeghai7thioyaiR4
Highly Voted 3 years, 7 months ago
Do not count the subnets sizes, we do not care What you need to know is: - the last digit is 2^x - the DNS server lives at the network IP addr + 2 So, we are looking for an addr that is equal to 2^x + 2 A: if there an x, where 2^x + 2 = 1 ? No -> false B: if there an x, where 2^x + 2 = 65 ? No -> false D: if there an x, where 2^x + 2 = 64 ? No -> false C: if there an x, where 2^x + 2 = 66 ? Yes, 2^6 + 2 = 66 -> that's is correct
upvoted 13 times
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JamesTR
Highly Voted 3 years, 8 months ago
Non of the above VPC CIDR is 172.16.12.0/24 so DNS IP is 172.16.12.2 BTW, there is one DNS per VPC, not one DNS per subnet.
upvoted 6 times
JamesTR
3 years, 7 months ago
After giving it some thought, I think the answer is C They are not asking what is the IP address of the DNS, but what IP is *reserved* for DNS in subnet 2. In any case, C is the best answer as all other answers are obviously wrong.
upvoted 1 times
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ChauPhan
Most Recent 3 years, 7 months ago
Can someone explain me? VPC CIDR block of 172.16.12.0/24 --> Totally 2^8 = 256 IPs, how can it be divide to 6 subnets with 64 IPs per subnet :(?
upvoted 1 times
jithin1234
3 years, 7 months ago
user this calculator to understand more https://www.site24x7.com/tools/ipv4-subnetcalculator.html 172.16.12.0/26 172.16.12.64/26= .65 is reserved for router, .66 is for DNS 172.16.12.128/26- subnet 3, from question.129 is reserved for VPC router 172.16.12.192/26
upvoted 1 times
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jimycrimson16
3 years, 7 months ago
subnet1 -> 172.16.12.0/26 subnet2 -> 172.16.12.64/26 subnet3 -> 172.16.12.128/27 subnet4 -> 172.16.12.160/27 subnet5 -> 172.16.12.192/27 subnet6 -> 172.16.12.224/27
upvoted 6 times
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awspro2021
3 years, 7 months ago
Subnet A: 172.16.12.0/26 [0-63] Subnet B: 172.16.12.64/26 [64-127] -> so the DNS IP is 172.16.64.65
upvoted 1 times
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doski
3 years, 8 months ago
The first two subnets is /26 (64 addresses) while the last four subnets has /27 (32 address). for the second subnet: Network address .64 Router address .65 DNS address .66 Ans:C
upvoted 6 times
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Paagee
3 years, 8 months ago
Why not 172.16.12.2? As stated, "The string AmazonProvidedDNS maps to a DNS server running on a reserved IP address at the base of the VPC IPv4 network range, plus two. For example, the DNS Server on a 10.0.0.0/16 network is located at 10.0.0.2. For VPCs with multiple IPv4 CIDR blocks, the DNS server IP address is located in the primary CIDR block. The DNS server does not reside within a specific subnet or Availability Zone in a VPC." https://docs.aws.amazon.com/vpc/latest/userguide/VPC_DHCP_Options.html#AmazonDNS
upvoted 2 times
sato_tomo
3 years, 8 months ago
10.0.0.2: Reserved by AWS. The IP address of the DNS server is the base of the VPC network range plus two. For VPCs with multiple CIDR blocks, the IP address of the DNS server is located in the primary CIDR. We also reserve the base of each subnet range plus two for all CIDR blocks in the VPC. Reference: https://docs.aws.amazon.com/vpc/latest/userguide/VPC_Subnets.html#VPC_Sizing
upvoted 2 times
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Souslik1999
3 years, 8 months ago
B - 172.16.64.65 is the correct answer. 172.16.64.64 /26 -> so the first usable IP is 172.16.64.65
upvoted 1 times
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