The answer is that the route table rtb-11111111 lacks a route to the S3 prefix list with the VPC endpoint as the target. A Gateway endpoint for S3 works by inserting a prefix list route into the associated route table, directing S3 traffic through the endpoint rather than over the internet. Simply associating the endpoint with a route table does not automatically add this route; the route must be explicitly present or added by the endpoint configuration. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding that Gateway endpoints rely on route table entries, not DNS or security groups, and a common trap is assuming association alone is sufficient. Remember the memory tip: “Associate the endpoint, but route the prefix list”—the endpoint association is just the link; the route table entry is the actual traffic director.
ANS-C01 Network Implementation Practice Question
This ANS-C01 practice question tests your understanding of network implementation. 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 VPC endpoint for S3 is created as a Gateway endpoint. The route tables rtb-11111111 and rtb-22222222 are associated with the endpoint. An EC2 instance in a subnet associated with rtb-11111111 cannot access S3 via 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
✓
The route table rtb-11111111 does not have a route to the S3 prefix list with the target as the VPC endpoint.
A Gateway endpoint automatically adds a prefix list route to the route table, but the route must point to the endpoint. If the route table already has a route to S3 via an Internet Gateway or NAT, the endpoint route may not be used. However, the most common issue is that the subnet's route table does not have the prefix list route for the S3 endpoint. The exhibit shows the endpoint is associated with the route table, but the route table itself must contain a route to the S3 prefix list (e.g., pl-12345) with the target as the endpoint. Option B (security group) is possible but less likely; Option C (DNS) is not required for Gateway endpoints; Option D (policy) is permissive.
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 route table rtb-11111111 does not have a route to the S3 prefix list with the target as the VPC endpoint.
Why this is correct
Without this route, traffic to S3 does not go through the endpoint.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The endpoint policy denies access to S3 from the instance's IAM role.
Why it's wrong here
The policy allows all principals and actions.
✗
The security group on the EC2 instance does not allow outbound traffic to the S3 prefix list.
Why it's wrong here
Security groups do not control traffic to a Gateway endpoint; they control traffic to IP addresses.
✗
The VPC does not have DNS resolution enabled for the endpoint.
Why it's wrong here
DNS resolution is not required for Gateway endpoints; they use prefix lists.
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 Implementation — This question tests Network Implementation — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The route table rtb-11111111 does not have a route to the S3 prefix list with the target as the VPC endpoint. — A Gateway endpoint automatically adds a prefix list route to the route table, but the route must point to the endpoint. If the route table already has a route to S3 via an Internet Gateway or NAT, the endpoint route may not be used. However, the most common issue is that the subnet's route table does not have the prefix list route for the S3 endpoint. The exhibit shows the endpoint is associated with the route table, but the route table itself must contain a route to the S3 prefix list (e.g., pl-12345) with the target as the endpoint. Option B (security group) is possible but less likely; Option C (DNS) is not required for Gateway endpoints; Option D (policy) is permissive.
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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