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 company has a three-tier application in a cloud VPC: web servers in a public subnet, application servers in a private subnet, and database servers in a private subnet. The web servers can connect to the application servers, but the application servers cannot connect to the database servers. The security groups are configured as follows:
- Web SG: inbound HTTP from 0.0.0.0/0, outbound all
- App SG: inbound HTTP from Web SG, outbound all
- DB SG: inbound MySQL from App SG, outbound all
What is the most likely cause of the connectivity issue?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The network access control list (NACL) on the database subnet is blocking inbound traffic from the application subnet.
Option D is correct because security groups are stateful and allow outbound traffic automatically, so outbound rules are not the issue. The inbound rule on DB SG allows MySQL from App SG, so A is not the issue. B is not needed because outbound is all. C is irrelevant. The issue likely is a NACL on the private subnet blocking the traffic, as NACLs are stateless and need explicit rules.
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 database security group is missing an inbound rule for MySQL.
Why it's wrong here
The DB SG has an inbound rule for MySQL from App SG, so this is not the issue.
✗
The application security group is missing an outbound rule for MySQL.
Why it's wrong here
Security groups are stateful; outbound traffic is automatically allowed if inbound is permitted.
✓
The network access control list (NACL) on the database subnet is blocking inbound traffic from the application subnet.
Why this is correct
NACLs are stateless, so they must have explicit rules for traffic. A missing rule can block connectivity.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The web security group is blocking traffic to the database.
Why it's wrong here
The web SG is not involved in app-to-db traffic.
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 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
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 CV0-004 subnetting questions on CIDR, address ranges, and subnet selection.
Troubleshooting — This question tests Troubleshooting — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The network access control list (NACL) on the database subnet is blocking inbound traffic from the application subnet. — Option D is correct because security groups are stateful and allow outbound traffic automatically, so outbound rules are not the issue. The inbound rule on DB SG allows MySQL from App SG, so A is not the issue. B is not needed because outbound is all. C is irrelevant. The issue likely is a NACL on the private subnet blocking the traffic, as NACLs are stateless and need explicit rules.
What should I do if I get this CV0-004 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 CV0-004 subnetting questions on CIDR, address ranges, and subnet selection.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
CIDR notation defines the prefix length.
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