- A
Use a NAT gateway in the public subnet for web tier, and route all traffic through it.
Why wrong: NAT gateway is for outbound, not inbound internet traffic.
- B
Use a VPN connection to the internet gateway and route traffic through a virtual private gateway.
Why wrong: Unnecessary complexity; internet gateway directly provides internet access.
- C
Use an internet gateway, public subnet for web tier, private subnets for app and database, and network ACLs to filter traffic between tiers.
Why wrong: NACLs are less flexible than security groups for stateful filtering between tiers.
- D
Use an internet gateway attached to the VPC, public subnet for web tier, private subnets for app and database tiers, and security groups to restrict traffic between tiers.
This standard architecture meets all requirements.
Security Groups for Tier Isolation in a VPC
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 designing a network for a three-tier web application. The web tier must be accessible from the internet, the application tier must only be accessible from the web tier, and the database tier must only be accessible from the application tier. All tiers must be in private subnets except the web tier. Which combination of AWS services and routing should be used to meet these requirements?
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Use an internet gateway attached to the VPC, public subnet for web tier, private subnets for app and database tiers, and security groups to restrict traffic between tiers.
Option D is correct because it uses an internet gateway attached to the VPC to provide internet access to the web tier in a public subnet, while the application and database tiers are placed in private subnets. Security groups are stateful and can restrict traffic between tiers by allowing inbound traffic only from the specific security group of the preceding tier (e.g., app tier SG allows traffic only from web tier SG), which meets the requirement without exposing private tiers to the internet. Network ACLs are stateless and less suitable for this granular, stateful filtering between tiers.
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.
- ✗
Use a NAT gateway in the public subnet for web tier, and route all traffic through it.
- ✗
Use a VPN connection to the internet gateway and route traffic through a virtual private gateway.
Why it's wrong here
Unnecessary complexity; internet gateway directly provides internet access.
- ✗
Use an internet gateway, public subnet for web tier, private subnets for app and database, and network ACLs to filter traffic between tiers.
Why it's wrong here
NACLs are less flexible than security groups for stateful filtering between tiers.
- ✓
Use an internet gateway attached to the VPC, public subnet for web tier, private subnets for app and database tiers, and security groups to restrict traffic between tiers.
Why this is correct
This standard architecture meets all requirements.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The ANS-C01 exam often tests the distinction between stateful security groups and stateless network ACLs, and the trap here is that candidates mistakenly choose network ACLs (Option C) because they think ACLs are the only way to filter traffic between subnets, overlooking that security groups can be applied to instances in private subnets and referenced by other security groups for cross-tier filtering.
Detailed technical explanation
How to think about this question
Security groups operate at the instance level and are stateful, meaning that if you allow inbound traffic from a source security group, the return traffic is automatically allowed regardless of outbound rules. This makes them ideal for multi-tier architectures where you can reference the web tier's security group as the source in the app tier's security group inbound rule, and similarly for the database tier. Under the hood, AWS uses a distributed firewall engine that evaluates security group rules before routing, ensuring that traffic between tiers is filtered without needing to manage ephemeral port ranges.
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.
Visual reference
What to study next
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FAQ
Questions learners often ask
What does this ANS-C01 question test?
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: Use an internet gateway attached to the VPC, public subnet for web tier, private subnets for app and database tiers, and security groups to restrict traffic between tiers. — Option D is correct because it uses an internet gateway attached to the VPC to provide internet access to the web tier in a public subnet, while the application and database tiers are placed in private subnets. Security groups are stateful and can restrict traffic between tiers by allowing inbound traffic only from the specific security group of the preceding tier (e.g., app tier SG allows traffic only from web tier SG), which meets the requirement without exposing private tiers to the internet. Network ACLs are stateless and less suitable for this granular, stateful filtering between tiers.
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.
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Last reviewed: Jul 4, 2026
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