The answer is that the VPC does not have DNS resolution enabled, or the endpoint’s private DNS option is disabled. This is correct because a VPC gateway endpoint for DynamoDB relies on DNS to resolve DynamoDB’s public hostnames to private IP addresses within the VPC; without this, the EC2 instance resolves DynamoDB to public IPs, forcing traffic over the internet instead of through the endpoint, even if the route table already contains the prefix list route. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of how gateway endpoints integrate with DNS settings—a common trap is assuming that adding the prefix list route alone guarantees connectivity, while forgetting that private DNS must be enabled for the endpoint to intercept traffic. A useful memory tip: “DNS first, route second”—without proper DNS resolution, the route to the prefix list never gets used.
ANS-C01 Network Design Practice Question
This ANS-C01 practice question tests your understanding of network design. 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.
A company has a VPC with a public subnet and a private subnet. An EC2 instance in the private subnet needs to access an Amazon DynamoDB table. The company wants to avoid sending traffic over the internet. A VPC gateway endpoint for DynamoDB is created and attached to the route table of the private subnet. The EC2 instance has an IAM role that grants access to DynamoDB. However, the application running on the instance cannot connect to DynamoDB. The network team confirms that the route table has a route to the DynamoDB prefix list. 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 VPC does not have DNS resolution enabled, or the endpoint's private DNS option is disabled.
Option D is correct because a VPC gateway endpoint for DynamoDB requires DNS resolution to resolve the DynamoDB public hostnames to private IP addresses within the VPC. If DNS resolution is disabled or the endpoint's private DNS option is disabled, the EC2 instance will resolve DynamoDB endpoints to public IPs, causing traffic to attempt internet routing rather than using the gateway endpoint. Since the route table already has the prefix list route, the most likely remaining issue is DNS-related.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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 on the EC2 instance blocks outbound traffic to DynamoDB.
The IAM role does not include the dynamodb:PutItem action.
Why it's wrong here
IAM role is correct.
✗
The route table does not have a route to the DynamoDB prefix list.
Why it's wrong here
Route is present.
✓
The VPC does not have DNS resolution enabled, or the endpoint's private DNS option is disabled.
Why this is correct
Correct: Without private DNS, the public endpoint is used, which routes over the internet.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
AWS often tests the misconception that a route to the prefix list alone is sufficient for gateway endpoint connectivity, ignoring the critical requirement for DNS resolution to redirect traffic through the endpoint.
Detailed technical explanation
How to think about this question
VPC gateway endpoints use AWS-managed prefix lists to route traffic via the endpoint's private IPs, but the instance must resolve DynamoDB DNS names (e.g., dynamodb.us-east-1.amazonaws.com) to those private IPs. If the endpoint's private DNS option is disabled, the instance will resolve to public IPs, and traffic will attempt to egress via an internet gateway or NAT device, failing if no such path exists. In real-world scenarios, this often occurs when the VPC's 'Enable DNS resolution' attribute is set to false, or when the endpoint is created without selecting 'Enable Private DNS Name'.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Network Design — This question tests Network Design — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The VPC does not have DNS resolution enabled, or the endpoint's private DNS option is disabled. — Option D is correct because a VPC gateway endpoint for DynamoDB requires DNS resolution to resolve the DynamoDB public hostnames to private IP addresses within the VPC. If DNS resolution is disabled or the endpoint's private DNS option is disabled, the EC2 instance will resolve DynamoDB endpoints to public IPs, causing traffic to attempt internet routing rather than using the gateway endpoint. Since the route table already has the prefix list route, the most likely remaining issue is DNS-related.
What should I do if I get this ANS-C01 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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?
Read the scenario before looking for a memorised answer.
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