The answer is that the subnet’s route table has a default route (0.0.0.0/0) pointing to an internet gateway, effectively making it a public subnet despite the engineer’s assumption. For an EC2 instance in a private subnet to achieve outbound internet access without a NAT gateway or instance, the only remaining mechanism for IPv4 traffic is a direct route to an internet gateway—but this contradicts the definition of a private subnet, which lacks such a route. On the AWS Certified Security Specialty SCS-C02 exam, this question tests your understanding of subnet routing fundamentals and the common misconfiguration where a subnet labeled “private” is actually public due to an incorrect route table entry. The trap is that engineers often overlook the route table when diagnosing outbound connectivity, assuming NAT is required. Memory tip: “No NAT, no public IP? Check the route—if 0.0.0.0/0 points to an IGW, your ‘private’ subnet is a wolf in sheep’s clothing.”
SCS-C02 Infrastructure Security Practice Question
This SCS-C02 practice question tests your understanding of infrastructure security. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. 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 notices that an EC2 instance in a private subnet is able to make outbound connections to the internet. The instance does not have a public IP, and there is no NAT gateway or instance in the VPC. 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 subnet's route table has a default route (0.0.0.0/0) pointing to an internet gateway.
An egress-only internet gateway is for IPv6, not IPv4. A VPC endpoint (Gateway type) for S3 or DynamoDB does not provide general internet access. The only remaining possibility is that the subnet's route table has a default route (0.0.0.0/0) pointing to an internet gateway, but the instance is in a private subnet? Actually, if it's a private subnet, there is no direct IGW. The most likely cause is an AWS managed NAT gateway? But the question says no NAT gateway. Another possibility: the instance has a public IP assigned? But it says no public IP. Let's re-evaluate: The stem says no public IP and no NAT gateway/instance. Possibly the VPC has a transit gateway or VPN? The answer might be that the VPC has an internet gateway attached and the subnet's route table has a default route to that IGW, but that would mean the subnet is public. However, the instance is in a private subnet (no direct route to IGW). The only way to get outbound internet without NAT is via an AWS service like a VPC endpoint for S3 or DynamoDB, but that's not general internet. The correct answer is likely: The instance is using an AWS managed NAT gateway (but the question says no NAT gateway). Hmm, maybe the instance has an Elastic IP? But no public IP. Let's think differently: Possibly the instance is behind a Network Load Balancer with internet-facing? No. The correct answer is: The VPC has an internet gateway and the subnet's route table has a default route to a NAT device that is not a NAT gateway (e.g., an EC2 NAT instance), but the question says no NAT instance. I think the intended answer is that the subnet's route table has a default route to an internet gateway (making it a public subnet), but the question says private subnet. Actually, the explanation should indicate that the subnet is actually public, but the engineer mistakenly thinks it's private. However, the answer options: Let's provide plausible options.
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 VPC has an egress-only internet gateway for IPv6 traffic.
Why it's wrong here
Egress-only internet gateways only work for IPv6, not IPv4.
✗
The instance has a public IP assigned automatically via Auto-assign Public IP.
Why it's wrong here
The question states no public IP.
✗
The instance is using a VPC endpoint (Gateway type) for S3.
Why it's wrong here
A VPC endpoint for S3 only provides access to S3, not general internet.
✓
The subnet's route table has a default route (0.0.0.0/0) pointing to an internet gateway.
Why this is correct
This would make the subnet effectively public, allowing outbound internet access.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
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 media company stores terabytes of video archives that are accessed once a year for audit purposes. Moving these objects to a cold storage tier (Azure Archive, S3 Glacier, or Google Nearline) costs a fraction of hot storage. Questions like this test whether you understand storage tiers, access frequency tradeoffs, and retrieval latency requirements.
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 subnet's route table has a default route (0.0.0.0/0) pointing to an internet gateway. — An egress-only internet gateway is for IPv6, not IPv4. A VPC endpoint (Gateway type) for S3 or DynamoDB does not provide general internet access. The only remaining possibility is that the subnet's route table has a default route (0.0.0.0/0) pointing to an internet gateway, but the instance is in a private subnet? Actually, if it's a private subnet, there is no direct IGW. The most likely cause is an AWS managed NAT gateway? But the question says no NAT gateway. Another possibility: the instance has a public IP assigned? But it says no public IP. Let's re-evaluate: The stem says no public IP and no NAT gateway/instance. Possibly the VPC has a transit gateway or VPN? The answer might be that the VPC has an internet gateway attached and the subnet's route table has a default route to that IGW, but that would mean the subnet is public. However, the instance is in a private subnet (no direct route to IGW). The only way to get outbound internet without NAT is via an AWS service like a VPC endpoint for S3 or DynamoDB, but that's not general internet. The correct answer is likely: The instance is using an AWS managed NAT gateway (but the question says no NAT gateway). Hmm, maybe the instance has an Elastic IP? But no public IP. Let's think differently: Possibly the instance is behind a Network Load Balancer with internet-facing? No. The correct answer is: The VPC has an internet gateway and the subnet's route table has a default route to a NAT device that is not a NAT gateway (e.g., an EC2 NAT instance), but the question says no NAT instance. I think the intended answer is that the subnet's route table has a default route to an internet gateway (making it a public subnet), but the question says private subnet. Actually, the explanation should indicate that the subnet is actually public, but the engineer mistakenly thinks it's private. However, the answer options: Let's provide plausible options.
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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