- 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.
Quick Answer
The answer is to place the DB instance in a private subnet and configure a security group to allow inbound traffic from the on-premises CIDR. This is the most secure approach because the traffic traverses the AWS Site-to-Site VPN, which is already a private, encrypted tunnel, so the database never needs public internet exposure. A security group acts as a stateful, host-level firewall, allowing you to restrict access precisely to the on-premises network range, whereas a network ACL is stateless and less granular. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this question tests your understanding of layered security within hybrid networking—specifically, how to secure RDS access from on-premises VPN private subnet without introducing unnecessary attack surfaces. A common trap is assuming a NAT gateway or public accessibility is needed for inbound database connections, but remember: NAT is for outbound only, and public access defeats the purpose of a VPN. Memory tip: “VPN means private, so keep it private—security group, not public group.”
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 C is correct. The most secure approach is to place the RDS instance in a private subnet with a security group that only allows inbound traffic from the on-premises network's CIDR range. Since the connection comes over the VPN (private network), it is not necessary to expose the database to the internet. Option A is incorrect because making the DB instance publicly accessible is a security risk. Option B is incorrect because a network ACL is stateless and less granular than a security group; also, allowing all traffic to the DB subnet is overly permissive. Option D is incorrect because a NAT gateway is for outbound traffic, not inbound.
Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
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.
Common exam traps
Common exam trap: usable hosts are not the same as total addresses
Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.
Detailed technical explanation
How to think about this question
Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.
KKey Concepts to Remember
- CIDR notation defines the prefix length.
- Block size helps identify subnet boundaries.
- Network and broadcast addresses are not usable hosts in normal IPv4 subnets.
- The required host count determines the smallest suitable subnet.
TExam Day Tips
- Write the block size before choosing the subnet.
- Check whether the question asks for hosts, subnets or a specific address range.
- Do not confuse /24, /25, /26 and /27 host counts.
Key takeaway
Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
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.
What to study next
Got this wrong? Here's your next step.
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related ANS-C01 subnetting questions on CIDR, address ranges, and subnet selection.
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FAQ
Questions learners often ask
What does this ANS-C01 question test?
Network Design — This question tests Network Design — CIDR notation defines the prefix length..
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 C is correct. The most secure approach is to place the RDS instance in a private subnet with a security group that only allows inbound traffic from the on-premises network's CIDR range. Since the connection comes over the VPN (private network), it is not necessary to expose the database to the internet. Option A is incorrect because making the DB instance publicly accessible is a security risk. Option B is incorrect because a network ACL is stateless and less granular than a security group; also, allowing all traffic to the DB subnet is overly permissive. Option D is incorrect because a NAT gateway is for outbound traffic, not inbound.
What should I do if I get this ANS-C01 question wrong?
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related ANS-C01 subnetting questions on CIDR, address ranges, and subnet selection.
What is the key concept behind this question?
CIDR notation defines the prefix length.
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Last reviewed: Jun 20, 2026
This ANS-C01 practice question is part of Courseiva's free Amazon Web Services certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the ANS-C01 exam.
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