- A
Place the web tier in a public subnet and the application tier in a private subnet, and configure security groups to allow inbound traffic to the application tier only from the web tier's security group.
This achieves isolation with minimal overhead.
- B
Create two VPCs: one for the web tier with a public subnet, and one for the application tier with a private subnet, and connect them using VPC peering.
Why wrong: VPC peering adds complexity; single VPC is simpler.
- C
Place the web tier in a public subnet and the application tier in a private subnet with a NAT gateway for outbound access, and use security groups to allow traffic from the web tier.
Why wrong: NAT gateway is unnecessary and adds cost; no outbound access required.
- D
Place both tiers in the same public subnet, and use a network ACL to restrict traffic from the web tier to the application tier.
Why wrong: Public subnets have direct internet access, violating isolation.
Isolating Application Tiers Using Security Groups and Private Subnets
This ANS-C01 practice question tests your understanding of network implementation. 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 web application on AWS. The web tier runs on EC2 instances behind an Application Load Balancer (ALB), and the application tier runs on EC2 instances that connect to an RDS MySQL Multi-AZ DB instance. The application tier must be isolated from the internet and only accessible from the web tier. Which network implementation meets these requirements with the LEAST administrative overhead?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue:
"least"Why it matters: You want the option with minimum overhead, fewest steps, or lowest impact — not the most feature-rich or comprehensive answer.
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
Place the web tier in a public subnet and the application tier in a private subnet, and configure security groups to allow inbound traffic to the application tier only from the web tier's security group.
Option A is correct because placing the web tier in a public subnet and the application tier in a private subnet, then using a security group rule that references the web tier's security group as the source, provides a stateful, application-layer firewall that allows traffic only from the web tier to the application tier. This design isolates the application tier from the internet while requiring no additional infrastructure like NAT gateways or VPC peering, minimizing administrative overhead. The security group reference automatically scales with the web tier's instances, eliminating the need to manage IP addresses.
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.
- ✓
Place the web tier in a public subnet and the application tier in a private subnet, and configure security groups to allow inbound traffic to the application tier only from the web tier's security group.
Why this is correct
This achieves isolation with minimal overhead.
Clue confirmation
The clue word "least" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Create two VPCs: one for the web tier with a public subnet, and one for the application tier with a private subnet, and connect them using VPC peering.
Why it's wrong here
VPC peering adds complexity; single VPC is simpler.
- ✗
Place the web tier in a public subnet and the application tier in a private subnet with a NAT gateway for outbound access, and use security groups to allow traffic from the web tier.
- ✗
Place both tiers in the same public subnet, and use a network ACL to restrict traffic from the web tier to the application tier.
Why it's wrong here
Public subnets have direct internet access, violating isolation.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often over-engineer the solution by adding unnecessary components like NAT gateways or VPC peering, misunderstanding that security group references can securely allow traffic from a public subnet to a private subnet within the same VPC without internet gateway or NAT involvement.
Detailed technical explanation
How to think about this question
Security group references work by using the ENI (Elastic Network Interface) of the source instances, not IP addresses, so traffic is allowed even if the web tier instances are replaced or scaled. Under the hood, AWS maps the security group ID to the private IPs of all instances in that group, updating the rule dynamically. In a real-world scenario, this design also supports cross-AZ traffic without additional configuration, as security groups are region-wide, and the private subnet's route table can point to a VPC endpoint for services like S3 if outbound access is later needed.
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 Implementation — This question tests Network Implementation — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Place the web tier in a public subnet and the application tier in a private subnet, and configure security groups to allow inbound traffic to the application tier only from the web tier's security group. — Option A is correct because placing the web tier in a public subnet and the application tier in a private subnet, then using a security group rule that references the web tier's security group as the source, provides a stateful, application-layer firewall that allows traffic only from the web tier to the application tier. This design isolates the application tier from the internet while requiring no additional infrastructure like NAT gateways or VPC peering, minimizing administrative overhead. The security group reference automatically scales with the web tier's instances, eliminating the need to manage IP addresses.
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.
Are there clue words in this question I should notice?
Yes — watch for: "least". You want the option with minimum overhead, fewest steps, or lowest impact — not the most feature-rich or comprehensive answer.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
Same concept, more angles
2 more ways this is tested on ANS-C01
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A company is running a two-tier application with a web tier and a database tier. The web tier must be accessible from the internet, but the database tier should only be accessible from the web tier. Which architecture should they use?
hard- ✓ A.Web tier in public subnet, database tier in private subnet with a security group allowing only web tier.
- B.Both tiers in public subnets with security groups restricting database access.
- C.Both tiers in private subnets with a NAT Gateway for internet access.
- D.Web tier in private subnet, database tier in public subnet with a security group.
Why A: The correct architecture places the web tier in a public subnet with an internet gateway for inbound traffic, and the database tier in a private subnet with a security group that only allows inbound traffic from the web tier's security group. This ensures the database is not directly reachable from the internet, adhering to the principle of least privilege and defense in depth. The security group acts as a stateful virtual firewall, allowing you to reference the web tier's security group as the source, which automatically adapts to any changes in the web tier's IP addresses.
Variation 2. A company has a multi-tier application deployed in a VPC. The web tier consists of an internet-facing Application Load Balancer (ALB) in public subnets, and EC2 instances in private subnets. The application tier runs on EC2 instances in separate private subnets, and the database tier uses an Amazon RDS for MySQL instance in private subnets. The application tier needs to connect to the database on port 3306. The security group for the RDS instance (sg-database) has an inbound rule allowing TCP 3306 from the security group of the application tier (sg-app). The application tier instances can connect to the database, but the web tier instances cannot. The web tier instances should not have direct database access. What is the most likely reason for the web tier's inability to connect to the database?
hard- A.The network ACL for the database subnet is blocking inbound traffic from the web tier subnets.
- ✓ B.The security group of the RDS instance does not have an inbound rule allowing traffic from the web tier's security group.
- C.The RDS instance is in a public subnet and requires a NAT Gateway for communication.
- D.The route table for the web tier subnets does not have a route to the database subnets.
Why B: The correct answer is B: The security group for the RDS instance (sg-database) only allows inbound traffic on port 3306 from the application tier's security group (sg-app). The web tier instances are in a different security group, so traffic from the web tier is implicitly denied. Option A is incorrect because network ACLs are stateless; if they were blocking web tier traffic, they would also affect the application tier, which can connect successfully. Option C is incorrect because the RDS instance is in private subnets and does not need a NAT Gateway; NAT Gateways are for outbound internet access. Option D is incorrect because route tables do not affect security group logic; routing only determines network path, not whether traffic is permitted by security groups.
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Last reviewed: Jul 4, 2026
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