- A
Disable the 'Publicly accessible' option for the RDS instances.
Why wrong: This is a good practice, but if the instance has a public IP via a subnet, it could still be accessible. The question asks for VPC-level actions.
- B
Create a network ACL that denies all inbound traffic from 0.0.0.0/0.
Why wrong: Network ACLs are stateless and would also affect outbound traffic; also, security groups are more appropriate.
- C
Remove the route to an internet gateway from the private subnet's route table.
Without a route to an internet gateway, traffic cannot reach the internet.
- D
Ensure that the RDS instances are launched in a private subnet without a public IP address.
Without a public IP, the instances are not directly reachable from the internet.
- E
Configure the security group for the RDS instances to deny all inbound traffic.
Why wrong: This would block all inbound traffic, but the question is about internet access; internal traffic may be needed.
RDS Security in Private Subnet — Prevent Internet Access | AWS Advanced Networking Specialty Explained
This ANS-C01 practice question tests your understanding of network design. 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 has a VPC with a public subnet and a private subnet. The private subnet hosts Amazon RDS instances. The security team wants to ensure that the RDS instances are not accessible from the internet. Which TWO actions should be taken?
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
Remove the route to an internet gateway from the private subnet's route table.
Options C and D are the correct actions. Removing the route to an internet gateway from the private subnet's route table ensures that the subnet cannot send traffic to or receive traffic from the internet. Launching RDS instances in a private subnet without a public IP address ensures they do not have direct internet connectivity. Option A is not effective because disabling 'Publicly accessible' only prevents RDS from being assigned a public IP address, but if the instance is in a public subnet with a public route, it could still be accessible. Option B is incorrect because a network ACL that denies all inbound traffic from 0.0.0.0/0 would block all inbound traffic, including necessary traffic from within the VPC, and is not a recommended approach. Option E is wrong because configuring a security group to deny all inbound traffic would block all inbound connections, which would prevent legitimate access to RDS from authorized resources.
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.
- ✗
Disable the 'Publicly accessible' option for the RDS instances.
Why it's wrong here
This is a good practice, but if the instance has a public IP via a subnet, it could still be accessible. The question asks for VPC-level actions.
- ✗
Create a network ACL that denies all inbound traffic from 0.0.0.0/0.
Why it's wrong here
Network ACLs are stateless and would also affect outbound traffic; also, security groups are more appropriate.
- ✓
Remove the route to an internet gateway from the private subnet's route table.
- ✓
Ensure that the RDS instances are launched in a private subnet without a public IP address.
Why this is correct
Without a public IP, the instances are not directly reachable from the internet.
Related concept
CIDR notation defines the prefix length.
- ✗
Configure the security group for the RDS instances to deny all inbound traffic.
Why it's wrong here
This would block all inbound traffic, but the question is about internet access; internal traffic may be needed.
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.
Visual reference
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: Remove the route to an internet gateway from the private subnet's route table. — Options C and D are the correct actions. Removing the route to an internet gateway from the private subnet's route table ensures that the subnet cannot send traffic to or receive traffic from the internet. Launching RDS instances in a private subnet without a public IP address ensures they do not have direct internet connectivity. Option A is not effective because disabling 'Publicly accessible' only prevents RDS from being assigned a public IP address, but if the instance is in a public subnet with a public route, it could still be accessible. Option B is incorrect because a network ACL that denies all inbound traffic from 0.0.0.0/0 would block all inbound traffic, including necessary traffic from within the VPC, and is not a recommended approach. Option E is wrong because configuring a security group to deny all inbound traffic would block all inbound connections, which would prevent legitimate access to RDS from authorized resources.
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.
About these practice questions
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Same concept, more angles
1 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 has a VPC with a public subnet and a private subnet. The private subnet contains Amazon RDS instances that should only be accessed by EC2 instances in the same VPC. The EC2 instances are in a security group named 'App-SG'. Which configuration will meet the requirement?
easy- A.Configure a network ACL on the private subnet to allow inbound traffic from the VPC CIDR.
- B.Configure a network ACL on the private subnet to allow inbound traffic from the public subnet CIDR.
- ✓ C.Configure a security group on the RDS instances to allow inbound traffic from the 'App-SG' security group.
- D.Configure a security group on the RDS instances to allow inbound traffic from the VPC CIDR.
Why C: Security groups are stateful and can reference other security groups as a source, allowing traffic from all instances associated with the source security group. By configuring the RDS security group to allow inbound traffic from 'App-SG', only EC2 instances in that group can access the RDS instances, meeting the requirement without exposing the database to the entire VPC CIDR.
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Last reviewed: Jun 20, 2026
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