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?
Network Topology
A
An inbound rule allowing traffic from the RDS endpoint address on port 3306.
Why wrong: Endpoint address is a DNS name, not a source IP.
B
An inbound rule allowing traffic from the EC2 instance's security group ID on port 3306.
This restricts access to only the EC2 instances in that security group.
C
An inbound rule allowing traffic from 10.0.0.0/8 on port 3306.
Why wrong: Too broad, allows any IP in the 10.0.0.0/8 range.
D
An inbound rule allowing traffic from the VPC CIDR (e.g., 10.0.0.0/16) on port 3306.
Why wrong: Allows any resource in the VPC, not just the EC2 instance.
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 B is correct because referencing the EC2 instance's security group ID in the RDS inbound rule is the most secure and specific method to restrict access to only that EC2 instance within the VPC. Option A is incorrect because the RDS endpoint is a DNS name for the database instance, not a source IP or security group. Option C is too broad, allowing any traffic from the 10.0.0.0/8 range, which may include subnets outside the VPC. Option D is less specific than using the security group ID and could permit access from any resource within the VPC CIDR, whereas the security group approach provides granular 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.
✗
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 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 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 B is correct because referencing the EC2 instance's security group ID in the RDS inbound rule is the most secure and specific method to restrict access to only that EC2 instance within the VPC. Option A is incorrect because the RDS endpoint is a DNS name for the database instance, not a source IP or security group. Option C is too broad, allowing any traffic from the 10.0.0.0/8 range, which may include subnets outside the VPC. Option D is less specific than using the security group ID and could permit access from any resource within the VPC CIDR, whereas the security group approach provides granular 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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