The answer is to add a route in the private subnet route table for the S3 prefix list pointing to the VPC endpoint. This is correct because a Gateway VPC endpoint functions as a route table target; without a specific route for the S3 prefix list, traffic destined for S3 follows the default route to the NAT Gateway and exits to the internet, bypassing the endpoint entirely. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of how Gateway endpoints differ from Interface endpoints—Gateway endpoints require explicit subnet route modifications, not security group or DNS changes. A common trap is assuming the NAT Gateway alone suffices or that the endpoint policy is the issue, but the missing route is the root cause. Remember the mnemonic: “Gateway needs a gateway route”—the endpoint itself is the target, not the NAT.
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.
A company has a VPC with multiple EC2 instances that need to access an Amazon S3 bucket. The network team wants to ensure that traffic to S3 stays within the AWS network and does not traverse the internet. The VPC has a VPC endpoint for S3 (Gateway type). The team has created the endpoint and attached the appropriate policy allowing access to the specific S3 bucket. However, EC2 instances in a private subnet cannot access the S3 bucket. The private subnet route table has a default route pointing to a NAT Gateway. Which change should the network team make to allow private instances to access S3 via the endpoint?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Add a route in the private subnet route table for the S3 prefix list (com.amazonaws.region.s3) pointing to the VPC endpoint.
For a Gateway VPC endpoint to work, the route table associated with the subnet must have a route that points to the endpoint for S3 (prefix list id). The existing default route to NAT Gateway would route traffic to the internet, not through the endpoint. Option A is correct because adding a route for the S3 prefix list to the endpoint overrides the default route for S3 traffic. Option D would cause asymmetric routing and potential issues.
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.
✗
Modify the VPC endpoint policy to allow all principals.
Why it's wrong here
Policy may already be correct; the issue is routing.
✗
Change the private subnet route table's default route to point to the VPC endpoint.
Why it's wrong here
Default route should point to NAT for internet access, not to the endpoint.
✗
Remove the default route from the private subnet route table.
Why it's wrong here
Removing default route would break internet access and cause asymmetric routing.
✓
Add a route in the private subnet route table for the S3 prefix list (com.amazonaws.region.s3) pointing to the VPC endpoint.
Why this is correct
This ensures S3 traffic uses the endpoint instead of the NATGateway.
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: Add a route in the private subnet route table for the S3 prefix list (com.amazonaws.region.s3) pointing to the VPC endpoint. — For a Gateway VPC endpoint to work, the route table associated with the subnet must have a route that points to the endpoint for S3 (prefix list id). The existing default route to NAT Gateway would route traffic to the internet, not through the endpoint. Option A is correct because adding a route for the S3 prefix list to the endpoint overrides the default route for S3 traffic. Option D would cause asymmetric routing and potential issues.
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.
What is the key concept behind this question?
CIDR notation defines the prefix length.
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Question Discussion
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