- A
Create a security group for the database that allows inbound from the on-premises network using a prefix list for 203.0.113.0/24. Create a VPC endpoint for S3 and allow outbound traffic to the endpoint. Keep the private subnet's network ACL default (allow all outbound).
Why wrong: Security groups do not support prefix lists for inbound rules from on-premises CIDRs; they only support CIDR blocks or security group IDs. Also, allowing all outbound traffic in the network ACL is not restrictive enough.
- B
Set up a Site-to-Site VPN between the on-premises network and the VPC. Modify the route tables to route 203.0.113.0/24 through the VPN. Place the database in a private subnet and allow inbound from the VPN CIDR. Use an internet gateway for outbound traffic to S3.
Why wrong: A VPN is overkill for a single IP range and adds complexity. Using an internet gateway for outbound traffic to S3 is not recommended; a VPC endpoint is more secure and cost-effective.
- C
Modify the database security group to allow inbound TCP 3306 from 203.0.113.0/24. Create a VPC endpoint for S3. Update the private subnet's network ACL to deny all outbound traffic except to the S3 VPC endpoint prefix list.
This directly permits the required on-premises access and restricts outbound traffic to only S3 via VPC endpoint, which is secure and efficient.
- D
Place the database in a public subnet and use a security group that allows inbound from 203.0.113.0/24. Use the NAT gateway for outbound traffic to S3. Configure the public subnet's network ACL to deny all outbound traffic except to S3.
Why wrong: Placing the database in a public subnet exposes it to the internet unnecessarily. The NAT gateway is not needed for on-premises access; also, network ACLs are stateless and require symmetric rules.
Quick Answer
The correct answer is to modify the database security group to allow inbound TCP 3306 from the on-premises CIDR 203.0.113.0/24, create a VPC endpoint for S3, and update the private subnet’s network ACL to deny all outbound traffic except to the S3 prefix list. This configuration directly addresses the hybrid on-premises access to private RDS by using a security group rule for inbound traffic, which is stateful and does not require a NAT gateway for return traffic, while the VPC endpoint enables private, secure connectivity to S3 without traversing the internet. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of layered security controls—specifically, the distinction between security groups (stateful, resource-level) and network ACLs (stateless, subnet-level)—and how to combine them for least-privilege access. A common trap is assuming a NAT gateway or internet gateway is needed for on-premises access via VPN or Direct Connect, but the correct approach uses direct security group rules. Memory tip: “SG for inbound, NACL for outbound, endpoint for services.”
ANS-C01 Network Security, Compliance and Governance Practice Question
This ANS-C01 practice question tests your understanding of network security, compliance and governance. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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 using Amazon VPC with a public subnet and a private subnet. The public subnet has a NAT gateway. The private subnet has an Amazon RDS for MySQL database. The security team wants to ensure that the database is not accessible from the internet, but they need to allow a specific on-premises network (IP range 203.0.113.0/24) to connect to the database for maintenance. The company also wants to restrict outbound traffic from the database to only necessary AWS services (e.g., S3 for backups) and deny all other outbound traffic. Which configuration should the security team implement?
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
Modify the database security group to allow inbound TCP 3306 from 203.0.113.0/24. Create a VPC endpoint for S3. Update the private subnet's network ACL to deny all outbound traffic except to the S3 VPC endpoint prefix list.
Option A is correct because modifying the database security group to allow inbound from the on-premises CIDR directly is the simplest and most secure way. Using a VPC endpoint for S3 allows private connectivity. A custom network ACL on the private subnet can deny all outbound traffic except to the VPC endpoint. Option B is wrong because a security group cannot reference another region's CIDR; also, allowing all outbound traffic to the VPC endpoint defeats the purpose. Option C is wrong because the NAT gateway is not needed for on-premises access; also, a security group cannot reference a prefix list for on-premises IPs. Option D is wrong because an internet gateway is not needed for on-premises access via VPN; also, allowing all outbound traffic to 0.0.0.0/0 is insecure.
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.
- ✗
Create a security group for the database that allows inbound from the on-premises network using a prefix list for 203.0.113.0/24. Create a VPC endpoint for S3 and allow outbound traffic to the endpoint. Keep the private subnet's network ACL default (allow all outbound).
- ✗
Set up a Site-to-Site VPN between the on-premises network and the VPC. Modify the route tables to route 203.0.113.0/24 through the VPN. Place the database in a private subnet and allow inbound from the VPN CIDR. Use an internet gateway for outbound traffic to S3.
- ✓
Modify the database security group to allow inbound TCP 3306 from 203.0.113.0/24. Create a VPC endpoint for S3. Update the private subnet's network ACL to deny all outbound traffic except to the S3 VPC endpoint prefix list.
Why this is correct
This directly permits the required on-premises access and restricts outbound traffic to only S3 via VPC endpoint, which is secure and efficient.
Related concept
CIDR notation defines the prefix length.
- ✗
Place the database in a public subnet and use a security group that allows inbound from 203.0.113.0/24. Use the NAT gateway for outbound traffic to S3. Configure the public subnet's network ACL to deny all outbound traffic except to S3.
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 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.
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 Security, Compliance and Governance — This question tests Network Security, Compliance and Governance — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: Modify the database security group to allow inbound TCP 3306 from 203.0.113.0/24. Create a VPC endpoint for S3. Update the private subnet's network ACL to deny all outbound traffic except to the S3 VPC endpoint prefix list. — Option A is correct because modifying the database security group to allow inbound from the on-premises CIDR directly is the simplest and most secure way. Using a VPC endpoint for S3 allows private connectivity. A custom network ACL on the private subnet can deny all outbound traffic except to the VPC endpoint. Option B is wrong because a security group cannot reference another region's CIDR; also, allowing all outbound traffic to the VPC endpoint defeats the purpose. Option C is wrong because the NAT gateway is not needed for on-premises access; also, a security group cannot reference a prefix list for on-premises IPs. Option D is wrong because an internet gateway is not needed for on-premises access via VPN; also, allowing all outbound traffic to 0.0.0.0/0 is insecure.
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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