The correct architecture places web servers in public subnets with an Internet Gateway and application servers in private subnets with a route to a NAT Gateway. This design works because the public subnet’s route table points directly to the Internet Gateway (IGW), allowing inbound internet traffic to reach the web tier, while the private subnet has no such direct route, preventing any unsolicited inbound connections from the internet. The NAT Gateway in a public subnet provides a one-way path for the application servers to initiate outbound connections—such as fetching updates—without exposing them to incoming traffic. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this multi-tier VPC public private subnet design tests your understanding of network segmentation and asymmetric routing; a common trap is assuming a NAT Gateway makes a subnet public, but it does not—only the route to an IGW defines a public subnet. A useful memory tip: “Public points out, private hides in—NAT lets them speak out, but never lets the world in.”
ANS-C01 Network Design Practice Question
This ANS-C01 practice question tests your understanding of network design. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A company is deploying a multi-tier application in a VPC. The web servers must be accessible from the internet, while the application servers must only be accessible from the web servers. Which architecture meets these requirements?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Web servers in public subnets with an Internet Gateway; application servers in private subnets with a route to the NAT Gateway.
Option D is correct because it places web servers in public subnets with an Internet Gateway (IGW) for direct internet access, while application servers reside in private subnets that lack a direct route to the IGW. The private subnets have a route to a NAT Gateway, allowing the application servers to initiate outbound internet connections (e.g., for updates) without being reachable from the internet. Security Groups on the application servers can further restrict inbound traffic to only the web servers' security group, ensuring the application tier is isolated.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
All servers in private subnets; use a load balancer in a public subnet.
Why it's wrong here
Web servers in private subnets cannot be directly accessed from the internet.
✗
Web servers in private subnets; application servers in public subnets with an Internet Gateway.
Why it's wrong here
Application servers would be exposed to the internet.
✗
All servers in public subnets; use Security Groups to restrict access.
Why it's wrong here
All servers would have public IPs and be potentially exposed.
✓
Web servers in public subnets with an Internet Gateway; application servers in private subnets with a route to the NAT Gateway.
Why this is correct
This provides internet access to web servers while keeping app servers private.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
AWS often tests the misconception that Security Groups alone can fully isolate traffic in a public subnet, but the trap here is that instances in public subnets still have public IP addresses and are reachable from the internet unless explicitly blocked by a Network ACL or Security Group, which is less reliable than using private subnets with no IGW route.
Detailed technical explanation
How to think about this question
Under the hood, a private subnet is defined by a route table that has no route to an Internet Gateway (0.0.0.0/0 -> igw-id), while a public subnet has such a route. The NAT Gateway enables outbound-only internet access for private instances by translating their private IPs to the NAT Gateway's Elastic IP, but inbound connections from the internet are blocked because the NAT Gateway does not accept unsolicited inbound traffic. In a real-world scenario, this architecture is critical for compliance frameworks like PCI DSS, where the application tier must be isolated from direct internet access to reduce the attack surface.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A media company stores terabytes of video archives that are accessed once a year for audit purposes. Moving these objects to a cold storage tier (Azure Archive, S3 Glacier, or Google Nearline) costs a fraction of hot storage. Questions like this test whether you understand storage tiers, access frequency tradeoffs, and retrieval latency requirements.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Network Design — This question tests Network Design — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Web servers in public subnets with an Internet Gateway; application servers in private subnets with a route to the NAT Gateway. — Option D is correct because it places web servers in public subnets with an Internet Gateway (IGW) for direct internet access, while application servers reside in private subnets that lack a direct route to the IGW. The private subnets have a route to a NAT Gateway, allowing the application servers to initiate outbound internet connections (e.g., for updates) without being reachable from the internet. Security Groups on the application servers can further restrict inbound traffic to only the web servers' security group, ensuring the application tier is isolated.
What should I do if I get this ANS-C01 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
About these practice questions
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Variation 1. A company is designing a VPC for a three-tier web application. The web servers must be accessible from the internet, while the application and database servers must be isolated. Which subnet design should the company use?
easy
A.A single subnet for all tiers with network ACLs to isolate traffic
✓ B.Public subnet for web servers, private subnets for application and database servers
C.Private subnets for all tiers
D.Public subnets for all tiers
Why B: Option A is correct because a public subnet for web servers allows internet access, while private subnets for application and database servers provide isolation. Option B is wrong because placing all servers in public subnets exposes them to the internet. Option C is wrong because placing all servers in private subnets would block internet access to web servers. Option D is wrong because a single subnet for all tiers does not provide isolation.
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Question Discussion
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