The answer is that the subnet’s route table does not have a route sending traffic to the AWS Network Firewall endpoint. This is the most likely cause of the traffic bypass because AWS Network Firewall’s stateful rule groups are only evaluated for traffic that is actually routed through the firewall endpoint; if the route table for the subnet containing the EC2 instance points to an internet gateway or a NAT gateway instead, the traffic never touches the firewall, and its deny rules are never applied. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of how VPC routing integrates with security services—a common trap is assuming that firewall policies apply to all VPC traffic automatically, when in fact explicit routing is required. Remember the memory tip: “No route, no firewall review”—if the packet’s path doesn’t include the firewall endpoint, stateful rules are invisible to that traffic.
ANS-C01 Network Security, Compliance and Governance Practice Question
This ANS-C01 practice question tests your understanding of network security, compliance and governance. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. 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 company's security team notices that a VPC flow log record shows an outbound connection from a private EC2 instance to an IP address in a restricted country. The security group allows outbound 0.0.0.0/0 for TCP 443. The network ACL allows outbound traffic to 0.0.0.0/0 on ephemeral ports. The company uses AWS Network Firewall with a firewall policy that has a stateful rule group that denies traffic to the restricted country. The flow log shows the traffic was accepted. 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.
Why wrong: VPC Flow Logs capture all accepted and rejected traffic that enters or leaves the VPC.
B
The subnet's route table does not have a route that sends traffic to the AWS Network Firewall endpoint.
If the route table does not point to the firewall, traffic bypasses the firewall and the stateful rule is not evaluated.
C
The network ACL inbound rule blocks the return traffic.
Why wrong: The network ACL allows outbound traffic on ephemeral ports, and the return traffic would be allowed by the inbound rule for established connections.
D
The security group outbound rule is not configured correctly.
Why wrong: The security group allows outbound HTTPS to 0.0.0.0/0, so the rule is correct.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The subnet's route table does not have a route that sends traffic to the AWS Network Firewall endpoint.
Option C is correct because AWS Network Firewall's stateful rules are evaluated only for traffic that goes through the firewall endpoint. If the subnet's route table does not point to the firewall endpoint, traffic bypasses the firewall. Option A is wrong because VPC Flow Logs capture traffic after security group and NACL evaluation. Option B is wrong because security group allows outbound HTTPS. Option D is wrong because network ACL allows outbound 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.
✗
VPC Flow Logs are not capturing all traffic.
Why it's wrong here
VPC Flow Logs capture all accepted and rejected traffic that enters or leaves the VPC.
✓
The subnet's route table does not have a route that sends traffic to the AWS Network Firewall endpoint.
Why this is correct
If the route table does not point to the firewall, traffic bypasses the firewall and the stateful rule is not evaluated.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The network ACL inbound rule blocks the return traffic.
Why it's wrong here
The network ACL allows outbound traffic on ephemeral ports, and the return traffic would be allowed by the inbound rule for established connections.
✗
The security group outbound rule is not configured correctly.
Why it's wrong here
The security group allows outbound HTTPS to 0.0.0.0/0, so the rule is correct.
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 Security, Compliance and Governance — This question tests Network Security, Compliance and Governance — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The subnet's route table does not have a route that sends traffic to the AWS Network Firewall endpoint. — Option C is correct because AWS Network Firewall's stateful rules are evaluated only for traffic that goes through the firewall endpoint. If the subnet's route table does not point to the firewall endpoint, traffic bypasses the firewall. Option A is wrong because VPC Flow Logs capture traffic after security group and NACL evaluation. Option B is wrong because security group allows outbound HTTPS. Option D is wrong because network ACL allows outbound 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.
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.
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 company has a VPC with multiple subnets. They have an AWS Network Firewall deployed in a firewall subnet. They want to inspect all outbound traffic from the VPC to the internet. Currently, the VPC route table has a default route (0.0.0.0/0) pointing to an internet gateway. What routing change is required to route outbound traffic through the firewall?
medium
A.Create a VPC Gateway Endpoint for the firewall service.
B.Add a NAT gateway in a public subnet and route 0.0.0.0/0 to the NAT gateway, then point the NAT gateway to the firewall.
✓ C.Add a route in the subnet route tables that sends 0.0.0.0/0 traffic to the Network Firewall endpoint's elastic network interface.
D.Add a transit gateway and attach the VPC and the internet gateway to it, then route traffic through the firewall.
Why C: Option A is correct because Network Firewall requires traffic to be routed through a firewall endpoint, which is done by adding a route to the firewall endpoint's elastic network interface. Option B is wrong because a NAT gateway is for outbound traffic but does not provide inspection. Option C is wrong because a transit gateway is for connecting VPCs, not for firewall routing. Option D is wrong because a VPC endpoint is for private access to services.
Variation 2. A company has a VPC with multiple subnets and uses AWS Network Firewall to inspect traffic. The security team notices that traffic to a specific IP (10.0.0.10) is being dropped unexpectedly. The firewall policy has a stateful rule group that allows all traffic. What is the MOST likely cause?
hard
A.The stateful rule group has an implicit deny for that IP
B.AWS Shield Advanced is blocking the traffic
✓ C.The stateless default action is set to drop
D.The subnet's route table does not have a route to the firewall endpoint
Why C: Option C is correct because Network Firewall evaluates stateless rules first, and a stateless default drop action would drop traffic before stateful rules are considered. Option A is wrong because stateful rules allow all. Option B is wrong because route table directs traffic to firewall. Option D is wrong because AWS Shield is for DDoS protection.
Variation 3. A company uses AWS Network Firewall to inspect traffic. They notice that some traffic is bypassing the firewall. The VPC has a route table with a default route (0.0.0.0/0) pointing to the firewall endpoint. Which configuration could cause traffic to bypass the firewall?
hard
A.The route table has route propagation enabled
B.The subnet is not associated with the route table
C.The firewall endpoint is in a different Availability Zone
✓ D.The route table has a more specific route for the destination traffic
Why D: If a subnet has a route for the specific destination that overrides the default route, traffic may bypass the firewall. Option A is wrong because more specific routes take precedence. Option B is wrong because subnet association does not cause bypass. Option C is wrong because firewall endpoint is in a different subnet. Option D is wrong because route propagation does not bypass.
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