The answer is that private DNS is not enabled. This is the most likely cause because when PrivateDnsEnabled is set to false, the default DNS resolution for S3 endpoints does not resolve to the endpoint’s private IP address, forcing traffic to traverse the internet instead of the VPC endpoint. Even with a correct route table and an available endpoint state, the EC2 instance cannot use the endpoint unless DNS queries for S3 return the endpoint’s IP, which requires enabling private DNS or using the endpoint-specific DNS name. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of how VPC endpoint private DNS interacts with route tables and endpoint policies—a common trap is assuming a route alone guarantees connectivity. Remember the mnemonic: “Route gets you there, but DNS tells you where.”
ANS-C01 Network Management and Operations Practice Question
This ANS-C01 practice question tests your understanding of network management and operations. 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.
Network Topology
Refer to the exhibit. A company has created a VPC endpoint for S3. However, an EC2 instance in the subnet associated with the route table cannot access S3 via the endpoint. The route table has a route to the endpoint. 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
✓
Private DNS is not enabled
The correct answer is C because the endpoint policy is set to Allow all, which is fine. However, the PrivateDnsEnabled is false, meaning that DNS resolution for S3 endpoints does not resolve to the endpoint IP. To use the endpoint, either enable Private DNS or use the endpoint-specific DNS name. Option A is wrong because the endpoint state is 'available'. Option B is wrong because the route table is associated. Option D is wrong because the security group is not specified (empty), but default SG allows all outbound 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 endpoint is in 'pending' state
Why it's wrong here
The endpoint state is 'available'.
✗
The route table is not associated with the subnet
Why it's wrong here
The route table is associated (RouteTableIds not empty).
✓
Private DNS is not enabled
Why this is correct
Without Private DNS, the instance does not resolve S3 to the endpoint IP.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
No security group is attached; default SG allows outbound.
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.
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 ANS-C01 subnetting questions on CIDR, address ranges, and subnet selection.
Network Management and Operations — This question tests Network Management and Operations — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: Private DNS is not enabled — The correct answer is C because the endpoint policy is set to Allow all, which is fine. However, the PrivateDnsEnabled is false, meaning that DNS resolution for S3 endpoints does not resolve to the endpoint IP. To use the endpoint, either enable Private DNS or use the endpoint-specific DNS name. Option A is wrong because the endpoint state is 'available'. Option B is wrong because the route table is associated. Option D is wrong because the security group is not specified (empty), but default SG allows all outbound traffic.
What should I do if I get this ANS-C01 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 ANS-C01 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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