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Exam ANS-C00 topic 1 question 96 discussion

Exam question from Amazon's ANS-C00
Question #: 96
Topic #: 1
[All ANS-C00 Questions]

A company is deploying a new web application that uses a three-tier model with a public-facing Network Load Balancer and web servers in an Amazon VPC. The application servers are hosted in the company's data center. There is an AWS Direct Connect connection between the VPC and the company's data center. Load testing results indicate that up to 100 servers, equally distributed across multiple Availability Zones, are required to handle peak loads.
The network engineer needs to design a VPC that has a /24 CIDR assigned to it.
How should the engineer allocate subnets across three Availability Zones for each tier?

  • A. Network Load Balancer: /29 per subnet Web: /26 per subnet
  • B. Network Load Balancer: /28 per subnet Web: /25 per subnet
  • C. Network Load Balancer: /28 per subnet Web: /27 per subnet
  • D. Network Load Balancer: /28 per subnet Web: /26 per subnet
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Suggested Answer: D 🗳️
by guruguru at May 14, 2020, 6:47 a.m.
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guruguru
Highly Voted 3 years, 6 months ago
D. /26 has 64 IPs, two of them sufficient for 100 servers. And NLB minimum is /28
upvoted 10 times
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walkwolf3
Highly Voted 3 years, 5 months ago
D Answer is correct, but explanations are not easy to understand. For each AZ, two subnets are required. One public subnet for LB, the minimum subnet size is /28, so for NLB, /28 subnet. 100 servers are needed in 3 AZs, they should be evenly distributed, so 100/3 = 34. Each AZ will need 34 servers minimum. Each subnet you need to take account of 5 AWS reserved IPs, so we need 34+5=39 IPs in each subnet. A /26 subnet can meet this requirement for Web. So the answer is D.
upvoted 8 times
sapien45
3 years, 1 month ago
great explanations , I always forgot the 5 reserved IPs
upvoted 1 times
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clooudy
2 years, 11 months ago
D is correct, nice explanation AZ1: /28 for NLB, /26 for servers AZ2: /28 for NLB, /26 for servers AZ3: /28 for NLB, /26 for servers /24 - divided into 3X/26s and 2X/28s ( left with one /28) distributed the app into 3 AZs
upvoted 1 times
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MaikM
Most Recent 3 years, 6 months ago
Can someone explain: I need three subnets to put EC2 instances there but why do I need fourth subnet for Network Load Balancer ?
upvoted 1 times
student2020
3 years, 5 months ago
Public subnet for NLB and private subnets for web servers
upvoted 1 times
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Stec1980
3 years, 6 months ago
D is correct, although best practise is that you should have a /27 subnet for Load Balancer's. In this case though 256 addresses in the /24, 3x /28's consumes 48, 3x /64 provides 3 lots of 64 (so 192) to use for the web tier, including space for further spikes (even with the usually AWS reserved addresses removed form use). A is incorrect because you can't provision /29 subnets in a VPC. B is incorrect because you can't fit the /25's into the remaining address space once you've allocated the /28 subnets. C is incorrect because it wouldn't give you enough addresses for the web servers to use.
upvoted 4 times
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Johnny_Green
3 years, 6 months ago
D is the correct answer. Note: "For internet-facing load balancers, the subnets that you specify must have at least 8 available IP addresses." Also note that "The CIDR block of a subnet can be the same as the CIDR block for the VPC (for a single subnet in the VPC), or a subset of the CIDR block for the VPC (for multiple subnets). The allowed block size is between a /28 netmask and /16 netmask." Taking these into consideration, /28 for NLB becomes a little more obvious.
upvoted 2 times
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SilverT
3 years, 6 months ago
D is correct
upvoted 2 times
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