- A
Enable VPC Flow Logs on the NAT gateway to detect unauthorized usage.
Why wrong: Flow logs are for monitoring, not blocking.
- B
Create an IAM policy that allows only the application instances to use the NAT gateway.
Why wrong: NAT gateways do not use IAM; they are network devices.
- C
Configure a security group for the NAT gateway that allows only the application instances' security group as source.
Why wrong: Security groups are stateful and can be applied to the NAT gateway's elastic network interface, but they control inbound traffic to the NAT, not outbound from instances.
- D
Configure a network ACL on the private subnet to allow outbound traffic only to the NAT gateway's private IP address.
A network ACL stateless rule can restrict outbound traffic to the NAT gateway's IP, preventing other traffic.
Quick Answer
The correct answer is to configure a network ACL on the private subnet to allow outbound traffic only to the NAT gateway's private IP address. This works because a network ACL acts as a stateless firewall at the subnet level, and by restricting the destination to the NAT gateway’s private IP, you ensure that only traffic destined for that specific gateway can leave the subnet, effectively limiting NAT gateway usage to authorized instances. On the AWS Certified Security Specialty SCS-C02 exam, this question tests your understanding of subnet-level traffic control versus instance-level controls; a common trap is assuming security groups can filter by destination, but they cannot reference a NAT gateway as a target. Another pitfall is confusing flow logs (monitoring only) with actual access controls. Memory tip: think of the network ACL as the bouncer at the subnet door—only traffic with the NAT gateway’s private IP on the guest list gets out.
SCS-C02 Infrastructure Security Practice Question
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 security engineer is designing a VPC with private subnets for an application that must access the internet for software updates. The VPC has a NAT gateway in a public subnet. The private subnet route table has a default route (0.0.0.0/0) pointing to the NAT gateway. Which additional security measure should be implemented to ensure that only the application instances can use the NAT gateway, and not any other resources in the VPC?
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Configure a network ACL on the private subnet to allow outbound traffic only to the NAT gateway's private IP address.
Option D is correct because using a VPC endpoint for the NAT gateway is not possible; a network ACL on the private subnet can restrict outbound traffic to the NAT gateway's IP, ensuring only that traffic can exit. Option A is wrong because security groups cannot reference the NAT gateway as a destination; they control inbound/outbound traffic based on IP/CIDR. Option B is wrong because there is no IAM policy for NAT gateway usage. Option C is wrong because a flow log does not block traffic; it only monitors.
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.
- ✗
Enable VPC Flow Logs on the NAT gateway to detect unauthorized usage.
Why it's wrong here
Flow logs are for monitoring, not blocking.
- ✗
Create an IAM policy that allows only the application instances to use the NAT gateway.
Why it's wrong here
NAT gateways do not use IAM; they are network devices.
- ✗
Configure a security group for the NAT gateway that allows only the application instances' security group as source.
- ✓
Configure a network ACL on the private subnet to allow outbound traffic only to the NAT gateway's private IP address.
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 SCS-C02 subnetting questions on CIDR, address ranges, and subnet selection.
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FAQ
Questions learners often ask
What does this SCS-C02 question test?
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: Configure a network ACL on the private subnet to allow outbound traffic only to the NAT gateway's private IP address. — Option D is correct because using a VPC endpoint for the NAT gateway is not possible; a network ACL on the private subnet can restrict outbound traffic to the NAT gateway's IP, ensuring only that traffic can exit. Option A is wrong because security groups cannot reference the NAT gateway as a destination; they control inbound/outbound traffic based on IP/CIDR. Option B is wrong because there is no IAM policy for NAT gateway usage. Option C is wrong because a flow log does not block traffic; it only monitors.
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.
What is the key concept behind this question?
CIDR notation defines the prefix length.
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Last reviewed: Jun 20, 2026
This SCS-C02 practice question is part of Courseiva's free Amazon Web Services certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the SCS-C02 exam.
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