This SCS-C02 practice question tests your understanding of infrastructure security. 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 company has a VPC with public and private subnets. The private subnets need to access the internet for software updates. The security engineer has set up a NAT gateway in a public subnet and updated the route tables accordingly. However, instances in the private subnets cannot reach the internet. The engineer checks the security group for the NAT gateway and finds that it allows all outbound traffic. What is the most likely cause of the issue?
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 security group for the NAT gateway does not allow inbound traffic from the private subnets.
Option D is correct because the NATgateway's security group must allow inbound traffic from the private subnets to receive return traffic. Option A is incorrect because the NAT gateway automatically responds. Option B is incorrect because network ACLs are stateless and require inbound rules for return traffic. Option C is incorrect because the private subnets' route table must have a route to the NAT gateway.
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 route table for the private subnet does not have a default route (0.0.0.0/0) pointing to the NAT gateway.
Why it's wrong here
This is necessary but not the most likely cause given the scenario.
✗
The NAT gateway does not have an Elastic IP address assigned.
Why it's wrong here
A NATgateway requires an Elastic IP for internet connectivity.
✓
The security group for the NAT gateway does not allow inbound traffic from the private subnets.
Why this is correct
The NATgateway's security group must allow inbound traffic from the private subnets to forward responses.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The network ACL for the private subnet does not allow inbound HTTP/HTTPS traffic.
Why it's wrong here
Network ACLs are stateless and need inbound rules for return traffic from the internet.
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.
Trap categories for this question
Scenario analysis trap
This is necessary but not the most likely cause given the scenario.
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 SCS-C02 subnetting questions on CIDR, address ranges, and subnet selection.
Infrastructure Security — This question tests Infrastructure Security — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The security group for the NAT gateway does not allow inbound traffic from the private subnets. — Option D is correct because the NAT gateway's security group must allow inbound traffic from the private subnets to receive return traffic. Option A is incorrect because the NAT gateway automatically responds. Option B is incorrect because network ACLs are stateless and require inbound rules for return traffic. Option C is incorrect because the private subnets' route table must have a route to the NAT gateway.
What should I do if I get this SCS-C02 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 SCS-C02 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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