- A
Assign a public IP address to the DB instance and restrict access using a security group.
Why wrong: Making the DB instance publicly accessible exposes it to the internet, which is less secure.
- B
Place the DB instance in a private subnet and use a NAT gateway to allow inbound traffic.
Why wrong: NAT gateways are for outbound traffic only; they do not support inbound connections from on-premises.
- C
Place the DB instance in a public subnet and configure a network ACL to allow traffic from the on-premises CIDR.
Why wrong: Network ACLs are stateless and less granular; public subnet also exposes the DB instance to the internet.
- D
Place the DB instance in a private subnet and configure a security group to allow inbound traffic from the on-premises CIDR.
Private subnet with security group provides granular, stateful firewall control without internet exposure.
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 has a VPC with public and private subnets. They launch an Amazon RDS for MySQL DB instance in a private subnet. The DB instance needs to be accessible from an on-premises application that connects via an AWS Site-to-Site VPN. What is the MOST secure way to allow the on-premises application to connect to the DB instance?
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 DB instance in a private subnet and configure a security group to allow inbound traffic from the on-premises CIDR.
Option D is correct because placing the RDS DB instance in a private subnet ensures it has no public IP address, and configuring a security group to allow inbound traffic from the on-premises CIDR over the Site-to-Site VPN provides a secure, direct connection without exposing the database to the internet. Security groups act as a stateful firewall at the instance level, allowing only specified traffic from the on-premises network through the VPN tunnel.
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.
- ✗
Assign a public IP address to the DB instance and restrict access using a security group.
Why it's wrong here
Making the DB instance publicly accessible exposes it to the internet, which is less secure.
- ✗
Place the DB instance in a private subnet and use a NAT gateway to allow inbound traffic.
Why it's wrong here
NAT gateways are for outbound traffic only; they do not support inbound connections from on-premises.
- ✗
Place the DB instance in a public subnet and configure a network ACL to allow traffic from the on-premises CIDR.
Why it's wrong here
Network ACLs are stateless and less granular; public subnet also exposes the DB instance to the internet.
- ✓
Place the DB instance in a private subnet and configure a security group to allow inbound traffic from the on-premises CIDR.
Why this is correct
Private subnet with security group provides granular, stateful firewall control without internet exposure.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse the directionality of NAT gateways (outbound only) or assume that placing a database in a public subnet with a network ACL is sufficient, overlooking the inherent security advantage of keeping the instance in a private subnet without public exposure.
Detailed technical explanation
How to think about this question
When using AWS Site-to-Site VPN, traffic from on-premises enters the VPC through the virtual private gateway and is routed to the private subnet based on the route table. Security groups evaluate traffic based on the source IP address (the on-premises CIDR) and are stateful, meaning return traffic is automatically allowed. In contrast, network ACLs are stateless and require separate rules for inbound and outbound traffic, making them more error-prone for this use case.
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 company's IT admin needs to give a contractor read-only access to production logs without sharing account credentials. Using role-based access control (RBAC) and temporary scoped permissions — not a permanent shared password — is the correct pattern. Questions like this test whether you can apply least-privilege access across cloud identity services.
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: Place the DB instance in a private subnet and configure a security group to allow inbound traffic from the on-premises CIDR. — Option D is correct because placing the RDS DB instance in a private subnet ensures it has no public IP address, and configuring a security group to allow inbound traffic from the on-premises CIDR over the Site-to-Site VPN provides a secure, direct connection without exposing the database to the internet. Security groups act as a stateful firewall at the instance level, allowing only specified traffic from the on-premises network through the VPN tunnel.
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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