The correct answer is to add an inbound rule referencing the EC2 instance’s security group ID on port 3306. This is the most secure and specific approach because security group rules in AWS are stateful and allow you to reference other security groups within the same VPC, enabling traffic based on the source instance’s membership rather than a fixed IP. On the AWS Certified Database Specialty DBS-C01 exam, this scenario tests your understanding of least-privilege access for RDS within a VPC, a common topic in the Security domain. A frequent trap is choosing a VPC CIDR rule, which would open the database to every resource in the VPC, including non-EC2 services, whereas referencing the EC2 security group restricts access to only that specific instance. Remember the memory tip: “Group your trust, not your IP”—when both resources live in the same VPC, always prefer security group IDs over CIDR blocks for granular control.
DBS-C01 Database Security Practice Question
This DBS-C01 practice question tests your understanding of database security. 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.
Network Topology
A developer needs to connect to the RDS instance from an EC2 instance in the same VPC. The EC2 instance's security group allows outbound traffic to 0.0.0.0/0. The RDS security group inbound rules currently allow traffic from 0.0.0.0/0 on port 3306. After a security review, the company decides to restrict inbound traffic to the VPC only. Which inbound rule should be added to the RDS security group?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
An inbound rule allowing traffic from the EC2 instance's security group ID on port 3306.
Option D is correct because the EC2 instance is in the same VPC, and using the security group ID is the most secure and specific. Option A is too broad. Option B is unnecessary since the RDS endpoint is not an IP. Option C would allow all traffic in the VPC, but using the security group is better.
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.
✗
An inbound rule allowing traffic from the RDS endpoint address on port 3306.
An inbound rule allowing traffic from 10.0.0.0/8 on port 3306.
Why it's wrong here
Too broad, allows any IP in the 10.0.0.0/8 range.
✗
An inbound rule allowing traffic from the VPC CIDR (e.g., 10.0.0.0/16) on port 3306.
Why it's wrong here
Allows any resource in the VPC, not just the EC2 instance.
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 healthcare organisation deploys an application with a public-facing web tier and a private database tier. The database subnet has no public IP and only accepts connections from the web tier's security group. Questions like this test whether you can design cloud network isolation using VNets/VPCs, subnets, and security group rules.
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 DBS-C01 subnetting questions on CIDR, address ranges, and subnet selection.
Database Security — This question tests Database Security — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: An inbound rule allowing traffic from the EC2 instance's security group ID on port 3306. — Option D is correct because the EC2 instance is in the same VPC, and using the security group ID is the most secure and specific. Option A is too broad. Option B is unnecessary since the RDS endpoint is not an IP. Option C would allow all traffic in the VPC, but using the security group is better.
What should I do if I get this DBS-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 DBS-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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Question Discussion
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