The answer is a route table conflict where the private subnet route table has a route for the on-premises CIDR pointing to the NAT Gateway instead of the Direct Connect virtual private gateway. This occurs because AWS route tables use the most specific route that matches the destination, so if a route for the on-premises network exists via the NAT Gateway, traffic intended for Direct Connect is incorrectly sent to the NAT Gateway and then to the internet, never reaching the on-premises network. On the AWS Certified Security Specialty SCS-C02 exam, this scenario tests your understanding of route priority and how overlapping routes can break hybrid connectivity—a common trap is assuming a NAT Gateway only handles internet traffic, but any route pointing to it will hijack traffic for that destination. Remember the memory tip: "NAT is for the net, not for the direct connect"—if a route to on-premises points to a NAT Gateway, traffic will be lost to the internet.
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.
An organization has a VPC with public and private subnets. A NAT Gateway is deployed in a public subnet to allow instances in private subnets to access the internet. The security team notices that instances in a private subnet can reach the internet, but cannot initiate connections to an on-premises network connected via AWS Direct Connect. The on-premises network advertises a specific route. 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 private subnet route table has a route for the on-premises CIDR pointing to the NAT Gateway.
Option A is correct because a more specific route to the on-premises network via the NATGateway may override the Direct Connect route in the route table. Option B is wrong because Security Groups do not affect outbound traffic initiated by instances. Option C is wrong because NACLs are stateless but would affect both directions. Option D is wrong because the internet gateway is not involved in Direct Connect traffic.
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 security group assigned to the instances does not allow outbound traffic to the on-premises network.
Why it's wrong here
Security groups are stateful; outbound traffic is allowed by default.
✗
The network ACL on the private subnet is blocking inbound traffic from the on-premises network.
Why it's wrong here
The instances are initiating outbound connections; inbound rules are irrelevant.
✓
The private subnet route table has a route for the on-premises CIDR pointing to the NAT Gateway.
Why this is correct
The NATGateway route may override the Direct Connect route.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Internet gateway is not used for Direct Connect traffic.
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 private subnet route table has a route for the on-premises CIDR pointing to the NAT Gateway. — Option A is correct because a more specific route to the on-premises network via the NAT Gateway may override the Direct Connect route in the route table. Option B is wrong because Security Groups do not affect outbound traffic initiated by instances. Option C is wrong because NACLs are stateless but would affect both directions. Option D is wrong because the internet gateway is not involved in Direct Connect traffic.
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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