The correct next step is to check the network ACL associated with the private subnet. This is because network ACLs are stateless, meaning they evaluate inbound and outbound traffic independently; even if the outbound rule allows traffic to the internet, the inbound rule must explicitly allow the return traffic from the NAT gateway’s elastic IP address. In contrast, security groups are stateful and automatically permit response traffic, so the already-allowed outbound rule is sufficient on that front, and the route table is correctly configured with a default route to the NAT gateway. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of the stateless versus stateful distinction between network ACLs and security groups, a common trap where candidates overlook the need for ephemeral port ranges in the inbound ACL rule. A helpful memory tip: “Stateful security groups track the conversation; stateless ACLs need both sides of the handshake.”
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 why an EC2 instance cannot communicate with 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 security group for the instance allows all outbound traffic. What should the engineer check NEXT?
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 associated with the private subnet
Option C is correct because network ACLs are stateless and must allow both inbound and outbound traffic for the response. Option A is wrong because security group already allows outbound. Option B is wrong because the route is configured. Option D is wrong because flow logs are for analysis, not a next step in troubleshooting connectivity.
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.
✗
Verify that the security group inbound rules allow return traffic
Why it's wrong here
Security groups are stateful, so inbound rules do not affect outbound-initiated traffic.
✗
Verify that the NAT gateway has an Elastic IP
Why it's wrong here
A NATgateway requires an EIP to communicate with the internet, but the route is already configured.
✓
Check the network ACL associated with the private subnet
Why this is correct
Network ACLs are stateless and must allow inbound ephemeral ports for return traffic.
Flow logs are useful for analysis but not a first troubleshooting step.
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: Check the network ACL associated with the private subnet — Option C is correct because network ACLs are stateless and must allow both inbound and outbound traffic for the response. Option A is wrong because security group already allows outbound. Option B is wrong because the route is configured. Option D is wrong because flow logs are for analysis, not a next step in troubleshooting connectivity.
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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Question Discussion
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