The correct answer is to configure a DB security group that allows inbound traffic from the EC2 security groups. This works because RDS security groups are stateful, instance-level firewalls that can reference other security groups as sources, enabling you to restrict RDS database access to specific EC2 instances without needing to know their IP addresses. On the AWS Certified Database Specialty DBS-C01 exam, this question tests your understanding of network-level access controls versus IAM or subnet-level mechanisms—a common trap is confusing security groups with network ACLs, which are stateless and apply to entire subnets. Remember that security groups are stateful and support logical references to other groups, while NACLs are stateless and require explicit rules for both inbound and outbound traffic. A useful memory tip: “Groups talk to groups” — EC2 security groups can be the source in an RDS security group rule, making instance-specific access simple and dynamic.
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.
A company is using Amazon RDS for MySQL and needs to restrict access to the database to only specific Amazon EC2 instances in the same VPC. Which security mechanism should be used?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Configure a DB security group that allows inbound traffic from the EC2 security groups.
Option A is correct because RDS security groups control inbound traffic at the instance level and can reference EC2 security groups. Option B is wrong because network ACLs are stateless and apply to subnets, not individual instances. Option C is wrong because IAM policies control API access, not network traffic. Option D is wrong because the DB subnet group defines subnet availability, not access control.
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.
✓
Configure a DB security group that allows inbound traffic from the EC2 security groups.
Why this is correct
RDS security groups can reference source security groups for fine-grained access.
Create an IAM policy that allows the EC2 instances to connect to the RDS instance.
Why it's wrong here
IAM policies do not control network access to the database.
✗
Create a DB subnet group that includes only the subnets where the EC2 instances reside.
Why it's wrong here
DB subnet groups determine availability, not access control.
✗
Modify the network ACL for the DB subnet to allow traffic from the EC2 instances' IP addresses.
Why it's wrong here
Network ACLs are stateless and apply at the subnet level, not instance-specific security.
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 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: Configure a DB security group that allows inbound traffic from the EC2 security groups. — Option A is correct because RDS security groups control inbound traffic at the instance level and can reference EC2 security groups. Option B is wrong because network ACLs are stateless and apply to subnets, not individual instances. Option C is wrong because IAM policies control API access, not network traffic. Option D is wrong because the DB subnet group defines subnet availability, not access control.
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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