- A
Check the network ACL of the subnet in VPC B where the target instance resides.
NACLs are stateless and may block inbound traffic.
- B
Confirm that both VPCs are in the same AWS account.
Why wrong: Cross-account VPC peering works; this is not a typical issue.
- C
Verify the VPC peering connection status is active.
Why wrong: Assuming route tables are correct, the peering status is likely active, so this is not a priority.
- D
Check the operating system firewall on the target instance.
Instance-level firewall can block traffic.
- E
Enable VPC Flow Logs on both VPCs to analyze traffic.
Why wrong: Flow Logs are helpful but not the first step; basic checks should be done first.
Question 1,460 of 1,705
ANS-C01 Network Management and Operations Practice Question
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 diagnosing a connectivity issue between two VPCs connected via VPC peering. The engineer has confirmed that the route tables in both VPCs have appropriate routes and the security groups allow traffic. However, traffic from VPC A to VPC B fails. Which TWO steps should the engineer take to troubleshoot? (Select TWO.)
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
✓
Check the network ACL of the subnet in VPC B where the target instance resides.
Option A is correct because checking the network ACL (NACL) of the subnet in VPC B where the target instance resides can reveal if inbound traffic is blocked by the stateless NACL. Option D is correct because verifying the operating system firewall on the target instance in VPC B can block traffic even if AWS network rules are open. Option B is wrong because VPC peering can work across different AWS accounts, so being in the same account is not necessary. Option C is wrong because the engineer should already have verified the peering connection status as part of initial checks; if it were inactive, the problem would be obvious, so it is not a likely next step. Option E is wrong because while VPC Flow Logs are useful for deep analysis, they are not the first step for a basic connectivity check; direct checks like NACLs and OS firewalls are more immediate.
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.
- ✓
Check the network ACL of the subnet in VPC B where the target instance resides.
Why this is correct
NACLs are stateless and may block inbound traffic.
Related concept
CIDR notation defines the prefix length.
- ✗
Confirm that both VPCs are in the same AWS account.
Why it's wrong here
Cross-account VPC peering works; this is not a typical issue.
- ✗
Verify the VPC peering connection status is active.
Why it's wrong here
Assuming route tables are correct, the peering status is likely active, so this is not a priority.
- ✓
Check the operating system firewall on the target instance.
- ✗
Enable VPC Flow Logs on both VPCs to analyze traffic.
Why it's wrong here
Flow Logs are helpful but not the first step; basic checks should be done first.
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: Check the network ACL of the subnet in VPC B where the target instance resides. — Option A is correct because checking the network ACL (NACL) of the subnet in VPC B where the target instance resides can reveal if inbound traffic is blocked by the stateless NACL. Option D is correct because verifying the operating system firewall on the target instance in VPC B can block traffic even if AWS network rules are open. Option B is wrong because VPC peering can work across different AWS accounts, so being in the same account is not necessary. Option C is wrong because the engineer should already have verified the peering connection status as part of initial checks; if it were inactive, the problem would be obvious, so it is not a likely next step. Option E is wrong because while VPC Flow Logs are useful for deep analysis, they are not the first step for a basic connectivity check; direct checks like NACLs and OS firewalls are more immediate.
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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