This CV0-004 practice question tests your understanding of 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.
A user reports that they cannot connect to a RDS database instance from their application. The security group for the RDS instance allows inbound traffic on port 3306 from the application server's security group. What should the administrator check NEXT?
IAM policy attached to the RDS instance
Why wrong: IAM controls API calls, not network connectivity.
Network ACL rules for the RDS subnet
Network ACLs act as a firewall at subnet level and can block inbound traffic.
Route table entries for the RDS subnet
Why wrong: Routing typically not the issue within same VPC.
Outbound security group rules on the RDS instance
Why wrong: Outbound rules are not relevant for inbound connections.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Network ACL rules for the RDS subnet
Option A is correct because network ACLs are stateless and may block traffic even if security groups allow it. Option B is wrong because SG outbound rules are typically allow all by default. Option C is wrong because route tables do not apply to traffic within a VPC for the same subnet? Actually, they do, but the problem is more likely at the ACL layer. Option D is wrong because IAM roles do not control network connectivity.
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
For each option: why learners choose it and why it is or isn't the right answer here.
IAM policy attached to the RDS instance
Why it's wrong here
IAM controls API calls, not network connectivity.
Network ACL rules for the RDS subnet
Route table entries for the RDS subnet
Why it's wrong here
Routing typically not the issue within same VPC.
Outbound security group rules on the RDS instance
Why it's wrong here
Outbound rules are not relevant for inbound connections.
Common exam traps
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
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.
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
A security administrator must allow nursing staff to reach a patient records server while blocking access from the guest Wi-Fi VLAN. After applying an extended ACL, traffic is still blocked from nursing workstations. The ACL was applied outbound instead of inbound on the wrong interface. Questions like this test ACL direction and placement rules.
What to study next
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related CV0-004 subnetting questions on CIDR, address ranges, and subnet selection.
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FAQ
Troubleshooting — This question tests Troubleshooting — CIDR notation defines the prefix length..
The correct answer is: Network ACL rules for the RDS subnet — Option A is correct because network ACLs are stateless and may block traffic even if security groups allow it. Option B is wrong because SG outbound rules are typically allow all by default. Option C is wrong because route tables do not apply to traffic within a VPC for the same subnet? Actually, they do, but the problem is more likely at the ACL layer. Option D is wrong because IAM roles do not control network connectivity.
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related CV0-004 subnetting questions on CIDR, address ranges, and subnet selection.
CIDR notation defines the prefix length.
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Last reviewed: Jun 24, 2026
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