The correct subnet design places web servers in public subnets with an Internet Gateway, and application and database servers in private subnets. This architecture directly satisfies the security requirement that the database tier must not be accessible from the internet, while the web tier remains reachable via HTTP/HTTPS. By isolating the application and database layers in private subnets, all east-west traffic between tiers flows over private IP addresses within the VPC, ensuring no direct internet path to sensitive data. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of layered security and subnet routing—a common trap is placing the database in a public subnet with restrictive security groups, which still exposes it to potential internet-based threats. The key concept is that a public subnet is defined by a route to an Internet Gateway, not by security group rules alone. Memory tip: think “web in the open, app and DB in the vault”—the Internet Gateway is the door only for the front-end tier.
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 designing a VPC for a three-tier application (web, application, database). The database tier should not be accessible from the internet, but the web tier must be accessible. Which subnet design should the network engineer use?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Place web servers in public subnets with an internet gateway, and app and database servers in private subnets.
Option C is correct because it places the web servers in a public subnet with an Internet Gateway (IGW) to allow inbound HTTP/HTTPS traffic from the internet, while the application and database servers are placed in private subnets with no direct internet path. This design ensures the database tier is isolated from the internet, meeting the security requirement, while the web tier remains accessible. The application servers can communicate with the database via private IP addresses within the VPC, and outbound internet access for the private subnets (if needed) can be provided through a NAT Gateway in a public subnet.
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 web and app servers in public subnets, and database servers in private subnets.
Why it's wrong here
App servers should not be directly exposed to the internet.
✗
Place all servers in private subnets and use a NAT Gateway for internet access.
Why it's wrong here
Private subnets cannot receive inbound traffic from the internet without a load balancer in a public subnet.
✓
Place web servers in public subnets with an internet gateway, and app and database servers in private subnets.
Why this is correct
This follows the traditional security model of isolation for backend tiers.
Related concept
Read the scenario before looking for a memorised answer.
✗
Place all servers in public subnets and rely on security group rules to restrict access.
Why it's wrong here
Public subnets have direct routes to the internet gateway, increasing attack surface.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume placing all servers in private subnets with a NATGateway provides inbound internet access, confusing outbound-only NAT with the inbound access required for the web tier, or they mistakenly think security groups alone can fully replace subnet-level isolation in a public subnet.
Detailed technical explanation
How to think about this question
In a three-tier VPC design, the web tier typically resides in a public subnet associated with a custom route table that has a default route (0.0.0.0/0) pointing to an Internet Gateway, while the app and database tiers reside in private subnets with routes that point only to a NAT Gateway or a VPC endpoint for outbound traffic. The database tier often uses security groups that allow inbound traffic only from the app tier’s security group, leveraging AWS’s security group referencing feature for fine-grained access control. A common real-world scenario is deploying a web application with an RDS database; the database must never have a public IP address, and its security group should explicitly deny inbound traffic from 0.0.0.0/0, relying on private subnet isolation and security group rules for defense in depth.
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: Place web servers in public subnets with an internet gateway, and app and database servers in private subnets. — Option C is correct because it places the web servers in a public subnet with an Internet Gateway (IGW) to allow inbound HTTP/HTTPS traffic from the internet, while the application and database servers are placed in private subnets with no direct internet path. This design ensures the database tier is isolated from the internet, meeting the security requirement, while the web tier remains accessible. The application servers can communicate with the database via private IP addresses within the VPC, and outbound internet access for the private subnets (if needed) can be provided through a NAT Gateway in a public subnet.
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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Question Discussion
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