The answer is the network ACL in the public subnet blocking inbound traffic to the NAT gateway. This is correct because while the private subnet’s route table correctly sends internet-bound traffic to the NAT gateway, the NAT gateway itself resides in a public subnet that must allow inbound return traffic from the internet to its Elastic IP. A network ACL in that public subnet, being stateless, needs an explicit inbound rule permitting this traffic; without it, the NAT gateway cannot receive responses, breaking connectivity. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of the stateless versus stateful boundary between security groups and NACLs, and it’s a common trap where candidates focus only on the private subnet’s route table or mistakenly apply a security group to the NAT gateway. Remember the key distinction: security groups are stateful and apply to instances, while NACLs are stateless and apply to subnets—so always check the public subnet’s NACL for inbound rules when troubleshooting NAT gateway connectivity.
ANS-C01 Network Security, Compliance and Governance Practice Question
This ANS-C01 practice question tests your understanding of network security, compliance and governance. 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 security engineer is troubleshooting connectivity issues. An EC2 instance cannot access the internet. The instance is in a private subnet with a route table that has a default route (0.0.0.0/0) pointing to a NAT gateway. The NAT gateway is in a public subnet with an Elastic IP. The security group allows all outbound traffic. 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.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The network ACL in the public subnet is blocking inbound traffic to the NAT gateway
Option A is correct because the network ACL in the public subnet must allow inbound traffic from the internet to the NATgateway's Elastic IP. Option B is wrong because the route table in the private subnet is correct. Option C is wrong because the NAT gateway's security group is not a thing; it uses security groups for instances. Option D is wrong because the internet gateway is required and assumed attached.
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.
✓
The network ACL in the public subnet is blocking inbound traffic to the NAT gateway
Why this is correct
NATgateway needs inbound allowance for return traffic.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The route table in the private subnet has no route to the NAT gateway
Why it's wrong here
Stem says route exists.
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 company's IT admin needs to give a contractor read-only access to production logs without sharing account credentials. Using role-based access control (RBAC) and temporary scoped permissions — not a permanent shared password — is the correct pattern. Questions like this test whether you can apply least-privilege access across cloud identity services.
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 network ACL in the public subnet is blocking inbound traffic to the NAT gateway — Option A is correct because the network ACL in the public subnet must allow inbound traffic from the internet to the NAT gateway's Elastic IP. Option B is wrong because the route table in the private subnet is correct. Option C is wrong because the NAT gateway's security group is not a thing; it uses security groups for instances. Option D is wrong because the internet gateway is required and assumed attached.
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.
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Question Discussion
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