← DOP-C02·Amazon Web Services

Question 118 of 1,750

Incident and Event Response →hardMultiple ChoiceObjective-mapped

Accessing S3 from Private Subnet EC2 Instances

This DOP-C02 practice question tests your understanding of incident and event response. 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 application running on an EC2 instance in a private subnet needs to access an S3 bucket. The instance has an IAM role with S3 access. However, the application is failing with timeout errors. The security group allows all outbound traffic, and the NACL allows outbound ephemeral ports. 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.

A
No VPC endpoint for S3
A VPC endpoint enables private connectivity to S3 from a private subnet.
B
Missing route in the route table to an Internet Gateway
Why wrong: Private subnets do not have routes to IGW, but S3 access via NAT or VPC endpoint is needed.
C
IAM role does not have correct trust policy
Why wrong: Trust policy is for assuming the role, not the issue here.
D
Missing HTTP proxy configuration
Why wrong: No proxy is required for S3 access.

Full breakdown with real-world context →

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

✓

No VPC endpoint for S3

Option A is correct because a VPC endpoint for S3 (Gateway or Interface) is needed for private subnet access to S3 without NAT. Without a VPC endpoint, the EC2 instance in a private subnet cannot reach S3, resulting in timeout errors. The security group and NACL settings are permissive, so they are not the issue. Option B is incorrect because the instance is in a private subnet; routing to an Internet Gateway is not necessary and would require a NAT device. Option C is incorrect because the IAM role has the necessary S3 permissions. Option D is incorrect because no HTTP proxy is required for S3 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.

✓
No VPC endpoint for S3
Why this is correct
A VPC endpoint enables private connectivity to S3 from a private subnet.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
CIDR notation defines the prefix length.
✗
Missing route in the route table to an Internet Gateway
Why it's wrong here
Private subnets do not have routes to IGW, but S3 access via NAT or VPC endpoint is needed.
✗
IAM role does not have correct trust policy
Why it's wrong here
Trust policy is for assuming the role, not the issue here.
✗
Missing HTTP proxy configuration
Why it's wrong here
No proxy is required for S3 access.

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 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.

Visual reference

Quick reference

AWS S3 Storage Class Comparison

Storage Class	Min Duration	Retrieval	Use Case
S3 Standard	None	Immediate	Frequently accessed data
S3 Standard-IA	30 days	Immediate	Infrequent access, rapid retrieval
S3 One Zone-IA	30 days	Immediate	Non-critical infrequent data
S3 Intelligent-Tiering	None	Immediate–hours	Unknown or changing access patterns
S3 Glacier Instant	90 days	Milliseconds	Archive with instant retrieval
S3 Glacier Flexible	90 days	Minutes–hours	Archive, flexible retrieval
S3 Glacier Deep Archive	180 days	Hours	Long-term compliance archive

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 DOP-C02 subnetting questions on CIDR, address ranges, and subnet selection.

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FAQ

Questions learners often ask

What does this DOP-C02 question test?

Incident and Event Response — This question tests Incident and Event Response — CIDR notation defines the prefix length..

What is the correct answer to this question?

The correct answer is: No VPC endpoint for S3 — Option A is correct because a VPC endpoint for S3 (Gateway or Interface) is needed for private subnet access to S3 without NAT. Without a VPC endpoint, the EC2 instance in a private subnet cannot reach S3, resulting in timeout errors. The security group and NACL settings are permissive, so they are not the issue. Option B is incorrect because the instance is in a private subnet; routing to an Internet Gateway is not necessary and would require a NAT device. Option C is incorrect because the IAM role has the necessary S3 permissions. Option D is incorrect because no HTTP proxy is required for S3 access.

What should I do if I get this DOP-C02 question wrong?

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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