The correct steps are to check the network ACL of the subnet in VPC B and verify the operating system firewall on the target instance. Even when VPC route tables and security groups are properly configured, traffic can still be silently dropped by a subnet-level network ACL that is blocking inbound traffic, or by the instance’s own OS firewall, such as iptables or Windows Firewall, which operates independently of AWS networking. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this question tests your understanding of the layered security model—security groups are stateful and allow return traffic, but NACLs are stateless and require explicit inbound and outbound rules, while OS firewalls act as a final gatekeeper. A common trap is assuming that if security groups and routes are correct, connectivity must work; instead, remember that NACLs and host-based firewalls are often overlooked. Memory tip: “Routes and SGs are not enough—check the NACL and the OS stuff.”
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 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 B is correct because checking the NACL of the subnet in VPC B where the target instance resides can reveal if inbound traffic is blocked. Option D is correct because verifying the OS-level firewall on the target instance in VPC B can block traffic even if AWS network is open. Option A is wrong because the peering connection status is active; if it were pending or deleted, it would be obvious. Option C is wrong because cross-account configuration is not mentioned. Option E is wrong because VPC Flow Logs are useful but not the first step for a basic connectivity check.
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.
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.
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.
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 B is correct because checking the NACL of the subnet in VPC B where the target instance resides can reveal if inbound traffic is blocked. Option D is correct because verifying the OS-level firewall on the target instance in VPC B can block traffic even if AWS network is open. Option A is wrong because the peering connection status is active; if it were pending or deleted, it would be obvious. Option C is wrong because cross-account configuration is not mentioned. Option E is wrong because VPC Flow Logs are useful but not the first step for a basic connectivity check.
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.
About these practice questions
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These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A network engineer is setting up a VPC peering connection between two VPCs (VPC-A and VPC-B) in different AWS accounts. The VPCs are in the same region. After accepting the peering request, instances in VPC-A cannot communicate with instances in VPC-B. What should the engineer check first?
medium
A.The security groups in both VPCs allow traffic from the peer VPC CIDR.
B.The network ACLs in both VPCs allow traffic from the peer VPC CIDR.
✓ C.The route tables in both VPCs have routes to the peer VPC CIDR.
D.The VPC peering connection status is 'active'.
Why C: Option C is correct because route tables must be updated with routes to the peer VPC CIDR. Without these routes, traffic is not directed to the peering connection. Option A is incorrect because security groups can be configured to allow cross-account traffic, but they are not the first check. Option B is incorrect because the peering connection is already accepted. Option D is incorrect because NACLs are not the primary issue; routes are.
Variation 2. Refer to the exhibit. A network engineer has established a VPC peering connection between VPC A (10.0.0.0/16) in account 111111111111 and VPC B (192.168.0.0/16) in account 222222222222. The peering connection status is 'active'. However, instances in VPC A cannot reach instances in VPC B. What is the MOST likely cause?
medium
A.The peering connection is not in the 'active' state
✓ B.Route tables in one or both VPCs do not have routes to the peer CIDR
C.Security groups in VPC B are blocking traffic
D.The CIDR blocks overlap
Why B: Option D is correct because the route tables in both VPCs need routes to the peer CIDR via the peering connection. Option A is incorrect because the status is active. Option B is incorrect because the CIDRs are non-overlapping. Option C is incorrect because there is no indication of security groups blocking, though they could; but the most common cause is missing routes.
Variation 3. A network engineer is setting up a VPC peering connection between two VPCs in the same AWS account and Region. Which TWO steps are required to enable communication between instances in the peered VPCs? (Choose two.)
easy
A.Attach an internet gateway to each VPC
B.Establish a VPN connection between the VPCs
✓ C.Add routes in each VPC's route table pointing to the CIDR of the other VPC
D.Configure a NAT gateway in each VPC
✓ E.Update security group rules to allow traffic from the peered VPC CIDR
Why C: Option A is correct because VPC peering requires adding routes in both VPC route tables. Option D is correct because security group rules must allow traffic from the peered VPC CIDR. Option B is wrong because VPC peering does not require an internet gateway. Option C is wrong because VPC peering does not use a VPN connection. Option E is wrong because a NAT gateway is not required for VPC peering.
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