- A
Review the inbound and outbound rules of the network ACL.
NACL rules explicitly allow or deny traffic.
- B
Check the security group rules for the affected instances.
Why wrong: Security groups are stateful and not directly related to NACL blocking.
- C
Use AWS CloudTrail to view network traffic logs.
Why wrong: CloudTrail logs API calls, not network traffic.
- D
Enable VPC Flow Logs and filter for 'ACCEPT' or 'REJECT' status.
Flow Logs can show whether NACL accepted or rejected packets.
- E
Test connectivity using AWS Direct Connect.
Why wrong: Direct Connect is for on-premises connectivity, not troubleshooting NACL.
Question 1,090 of 1,705
Verify if NACL is Blocking Traffic
This ANS-C01 practice question tests your understanding of network management and operations. 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 network engineer is troubleshooting network connectivity issues in a VPC. The engineer suspects that the network ACL is blocking traffic. Which TWO actions should the engineer take to verify this?
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Review the inbound and outbound rules of the network ACL.
Options A and D are correct because reviewing the inbound and outbound rules of the network ACL allows the engineer to check if traffic is explicitly blocked by the ACL, and enabling VPC Flow Logs with 'ACCEPT' or 'REJECT' status provides insight into whether the ACL is actually blocking the traffic. Option B is incorrect because security groups are stateful and work at the instance level, not at the subnet level like NACLs. Option C is incorrect because AWS CloudTrail records API activity, not network traffic. Option E is incorrect because AWS Direct Connect establishes a dedicated network connection but does not help verify if a NACL is blocking traffic.
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.
- ✓
Review the inbound and outbound rules of the network ACL.
Why this is correct
NACL rules explicitly allow or deny traffic.
Related concept
CIDR notation defines the prefix length.
- ✗
Check the security group rules for the affected instances.
Why it's wrong here
Security groups are stateful and not directly related to NACL blocking.
- ✗
Use AWS CloudTrail to view network traffic logs.
Why it's wrong here
CloudTrail logs API calls, not network traffic.
- ✓
Enable VPC Flow Logs and filter for 'ACCEPT' or 'REJECT' status.
Why this is correct
Flow Logs can show whether NACL accepted or rejected packets.
Related concept
CIDR notation defines the prefix length.
- ✗
Test connectivity using AWS Direct Connect.
Why it's wrong here
Direct Connect is for on-premises connectivity, not troubleshooting NACL.
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 healthcare organisation deploys an application with a public-facing web tier and a private database tier. The database subnet has no public IP and only accepts connections from the web tier's security group. Questions like this test whether you can design cloud network isolation using VNets/VPCs, subnets, and security group rules.
Visual reference
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 ANS-C01 subnetting questions on CIDR, address ranges, and subnet selection.
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FAQ
Questions learners often ask
What does this ANS-C01 question test?
Network Management and Operations — This question tests Network Management and Operations — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: Review the inbound and outbound rules of the network ACL. — Options A and D are correct because reviewing the inbound and outbound rules of the network ACL allows the engineer to check if traffic is explicitly blocked by the ACL, and enabling VPC Flow Logs with 'ACCEPT' or 'REJECT' status provides insight into whether the ACL is actually blocking the traffic. Option B is incorrect because security groups are stateful and work at the instance level, not at the subnet level like NACLs. Option C is incorrect because AWS CloudTrail records API activity, not network traffic. Option E is incorrect because AWS Direct Connect establishes a dedicated network connection but does not help verify if a NACL is blocking traffic.
What should I do if I get this ANS-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 ANS-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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Last reviewed: Jun 20, 2026
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