The correct answer is an egress-only internet gateway, which is used for IPv6 traffic. This is because an egress-only internet gateway enables outbound-only communication from a private subnet to the internet, while blocking any unsolicited inbound connections—a critical distinction from an internet gateway, which allows bidirectional traffic. On the AWS Certified Security Specialty SCS-C02 exam, this question tests your understanding of how IPv6 routing differs from IPv4; a common trap is assuming a NAT gateway is required for all internet access, but NAT gateways only support IPv4. The scenario often appears when an engineer sees outbound traffic succeeding without a NAT device, and the key clue is the presence of an IPv6 address on the EC2 instance. Memory tip: think of the egress-only gateway as a one-way valve for IPv6—outbound only, no return fire.
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 can reach the internet, even though there is no NAT gateway or instance in the route table. 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
✓
An egress-only internet gateway is used for IPv6 traffic.
An egress-only internet gateway allows IPv6 traffic to the internet but not inbound. If the instance has an IPv6 address and the route table points to an egress-only internet gateway, it can reach the internet. Option D is correct. NAT gateways (A) are for IPv4. Internet gateway (B) would allow inbound. VPC endpoint (C) does not provide internet access.
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.
✗
An internet gateway is attached to the VPC and a default route points to it.
Why it's wrong here
Internet gateway would allow inbound, but instance is in private subnet.
✗
A VPC endpoint for S3 is configured.
Why it's wrong here
VPC endpoint provides access to S3 only, not general internet.
✗
A NAT gateway is configured in a different availability zone.
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: An egress-only internet gateway is used for IPv6 traffic. — An egress-only internet gateway allows IPv6 traffic to the internet but not inbound. If the instance has an IPv6 address and the route table points to an egress-only internet gateway, it can reach the internet. Option D is correct. NAT gateways (A) are for IPv4. Internet gateway (B) would allow inbound. VPC endpoint (C) does not provide internet access.
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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