The answer is that the RDS instance is in a private subnet and does not have a public IP address. This is correct because an EC2 instance attempting to connect to an RDS database must be able to reach it over the network; if the RDS instance resides in a private subnet without a public IP, it cannot be accessed from outside that subnet’s VPC, even if security groups and network ACLs are permissive. On the AWS Certified Database Specialty DBS-C01 exam, this scenario tests your understanding of network architecture fundamentals—specifically, that RDS instances launched in private subnets are only reachable from within the same VPC or via a bastion host, and that a public IP is required for internet-based connectivity. A common trap is to focus solely on security group rules or ACLs while overlooking the subnet placement and public IP assignment. Memory tip: “Private subnet, no public IP? No internet path, no connectivity.”
DBS-C01 Monitoring and Troubleshooting Practice Question
This DBS-C01 practice question tests your understanding of monitoring and troubleshooting. 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.
An application running on Amazon EC2 is unable to connect to an Amazon RDS for SQL Server DB instance. The security group for the RDS instance allows inbound traffic from the security group of the EC2 instance on port 1433. The network ACLs allow all traffic. What is a likely cause of the connectivity issue?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The RDS instance is in a private subnet and does not have a public IP address
Option C is correct because RDS instances are not accessible from the internet by default; they must be launched in a public subnet with a public IP. Option A is wrong because security groups are properly configured. Option B is wrong because network ACLs allow all traffic. Option D is wrong because port 1433 is the default SQL Server port.
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.
✓
The RDS instance is in a private subnet and does not have a public IP address
Why this is correct
Without a public IP, the EC2 instance cannot reach it over the internet.
The security group inbound rule is incorrectly configured
Why it's wrong here
The security group allows traffic from the EC2 security group.
✗
The database port is not 1433
Why it's wrong here
SQL Server default port is 1433.
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.
Monitoring and Troubleshooting — This question tests Monitoring and Troubleshooting — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The RDS instance is in a private subnet and does not have a public IP address — Option C is correct because RDS instances are not accessible from the internet by default; they must be launched in a public subnet with a public IP. Option A is wrong because security groups are properly configured. Option B is wrong because network ACLs allow all traffic. Option D is wrong because port 1433 is the default SQL Server port.
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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