The correct answer is to add a network ACL rule allowing inbound TCP/80 from 10.0.1.5 to the subnet of 10.0.2.10. This is because the VPC flow log entry shows a REJECT, which is the signature behavior of a stateless network ACL—unlike security groups, which are stateful and silently drop traffic without generating a REJECT at the network layer. When troubleshooting VPC flow log REJECT due to NACL, remember that NACLs evaluate rules in order and block traffic unless an explicit allow rule matches the source, destination, and port. On the CompTIA Cloud+ CV0-004 exam, this scenario tests your ability to distinguish between stateful security groups and stateless NACLs, a common trap where candidates mistakenly blame security groups or routing. A quick memory tip: “REJECT equals stateless—check the NACL first; ACCEPT equals stateful—check the security group.”
CV0-004 Troubleshooting Practice Question
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.
Exhibit
Refer to the exhibit.
VPC Flow Logs excerpt:
2 123456789010 eni-12345678 10.0.1.5 10.0.2.10 12345 80 6 1500 100 1625082000 1625082060 ACCEPT OK
2 123456789010 eni-12345678 10.0.1.5 10.0.2.10 12346 80 6 1500 200 1625082061 1625082121 ACCEPT OK
2 123456789010 eni-12345678 10.0.1.5 10.0.2.10 12347 80 6 100 0 1625082122 1625082182 REJECT OK
The exhibit shows VPC flow log entries for an EC2 instance (eni-12345678). The administrator is troubleshooting a connectivity problem where an application on 10.0.1.5 occasionally cannot connect to a web server at 10.0.2.10 on port 80. What action should the administrator take?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Add a network ACL rule allowing inbound TCP/80 from 10.0.1.5 to the subnet of 10.0.2.10.
Option B is correct because the REJECT entry indicates that traffic is being blocked by a stateless firewall (NACL) because security groups are stateful and would not generate REJECT at network level. The REJECT is from a NACL. Adding an inbound allow rule in NACL for the target subnet would fix it. Option A is wrong because security groups are stateful and allow return traffic; the initial connection may be allowed but not specified; but flow logs show REJECT which is typical from NACL. Option C is wrong because routes are for routing, not filtering. Option D is wrong because MTU doesn't cause reject.
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.
✗
Increase the MTU on the instance to 9001.
Why it's wrong here
MTU issues would cause different symptoms, not REJECT.
✗
Add a security group rule allowing inbound TCP/80 from 10.0.1.5.
Why it's wrong here
Security groups are stateful and allow return traffic; the REJECT in flow logs indicates a NACL, not a security group.
✓
Add a network ACL rule allowing inbound TCP/80 from 10.0.1.5 to the subnet of 10.0.2.10.
Why this is correct
NACLs are stateless and require explicit inbound rules; the REJECT indicates a NACL block.
Routes determine the path, not filtering; the flow logs show traffic reaching the target, so routing is fine.
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.
Trap categories for this question
Command / output trap
Routes determine the path, not filtering; the flow logs show traffic reaching the target, so routing is fine.
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: Add a network ACL rule allowing inbound TCP/80 from 10.0.1.5 to the subnet of 10.0.2.10. — Option B is correct because the REJECT entry indicates that traffic is being blocked by a stateless firewall (NACL) because security groups are stateful and would not generate REJECT at network level. The REJECT is from a NACL. Adding an inbound allow rule in NACL for the target subnet would fix it. Option A is wrong because security groups are stateful and allow return traffic; the initial connection may be allowed but not specified; but flow logs show REJECT which is typical from NACL. Option C is wrong because routes are for routing, not filtering. Option D is wrong because MTU doesn't cause reject.
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.
What is the key concept behind this question?
CIDR notation defines the prefix length.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
This CV0-004 practice question is part of Courseiva's free CompTIA certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the CV0-004 exam.
Question Discussion
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
Sign in to join the discussion.