The answer is a misconfigured network ACL blocking all inbound traffic. This is because a network ACL operates at the subnet level as a stateless firewall, meaning it evaluates inbound and outbound traffic independently, and an explicit deny rule for all inbound traffic will override any permissive security group rules attached to the instance. On the CompTIA Cloud+ CV0-004 exam, this scenario tests your understanding of the layered defense model, where security groups are stateful and allow return traffic automatically, while network ACLs require explicit rules for both directions. A common trap is assuming a permissive security group alone guarantees connectivity, but the subnet’s network ACL acts as the first line of defense for inbound traffic. Remember the memory tip: “ACLs are stateless, so they block without a matching allow; SGs are stateful, so they remember the flow.”
CV0-004 Operations and Support Practice Question
This CV0-004 practice question tests your understanding of operations and support. 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.
Network Topology
Refer to the exhibit. A cloud administrator is troubleshooting why an EC2 instance in subnet-12345678 cannot be reached from the internet. The instance has a public IP and the security group allows all inbound traffic. Based on the exhibit, what is the most likely cause?
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 ACL is misconfigured and blocking all inbound traffic.
Option B is correct. The network ACL associated with the subnet has an inbound deny rule for all traffic, blocking access despite the security group. Options A, C, and D are not supported by the exhibit.
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 security group is not applied.
Why it's wrong here
The exhibit does not show security group association; the problem states the security group allows all inbound, so it is likely applied.
✗
The route table is missing a default route to the internet gateway.
Why it's wrong here
The exhibit does not show route tables; it shows a network ACL with a deny rule.
✗
The instance is stopped.
Why it's wrong here
The exhibit does not show instance state; the problem statement says the instance has a public IP, implying it is running.
✓
The network ACL is misconfigured and blocking all inbound traffic.
Why this is correct
The network ACL has a deny rule for all inbound traffic, which overrides the security group's allow.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
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.
Trap categories for this question
Command / output trap
The exhibit does not show security group association; the problem states the security group allows all inbound, so it is likely applied.
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.
Operations and Support — This question tests Operations and Support — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The network ACL is misconfigured and blocking all inbound traffic. — Option B is correct. The network ACL associated with the subnet has an inbound deny rule for all traffic, blocking access despite the security group. Options A, C, and D are not supported by the exhibit.
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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