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 a CIDR of 10.0.0.0/16. They have two public subnets (10.0.1.0/24 and 10.0.2.0/24) and two private subnets (10.0.3.0/24 and 10.0.4.0/24). They have an Application Load Balancer in the public subnets and EC2 instances in the private subnets. The EC2 instances need to access the internet for updates. The security engineer has set up a NAT gateway in each public subnet. The route table for the private subnets has a default route pointing to the NAT gateway in the same Availability Zone. However, the EC2 instances are unable to reach the internet. 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 route tables for the public subnets do not have a default route (0.0.0.0/0) to the Internet Gateway.
Option C is correct. Each NAT gateway must be in a public subnet with a route to the Internet Gateway. If the route table for the public subnet where the NAT gateway resides does not have a default route to the IGW, the NAT gateway cannot forward traffic. Option A is incorrect because each NAT gateway can be in a different AZ. Option B is incorrect because route propagation is for VPN. Option D is incorrect because security groups allow outbound by default.
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 tables for the public subnets do not have a default route (0.0.0.0/0) to the Internet Gateway.
Why this is correct
NAT gateways need a route to IGW to reach the internet.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The NAT gateways are in the same Availability Zone.
Why it's wrong here
They can be in different AZs; this is not a problem.
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.
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 route tables for the public subnets do not have a default route (0.0.0.0/0) to the Internet Gateway. — Option C is correct. Each NAT gateway must be in a public subnet with a route to the Internet Gateway. If the route table for the public subnet where the NAT gateway resides does not have a default route to the IGW, the NAT gateway cannot forward traffic. Option A is incorrect because each NAT gateway can be in a different AZ. Option B is incorrect because route propagation is for VPN. Option D is incorrect because security groups allow outbound by default.
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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