- A
The route table for private subnets now has a more specific route for S3 that overrides the default route to the NAT gateway
The endpoint adds a route for the S3 prefix list, but if the prefix list includes 0.0.0.0/0 erroneously, or if the endpoint route is not properly scoped, it can cause issues. More commonly, the endpoint adds a route for the S3 service (e.g., com.amazonaws.<region>.s3) which does not affect the default route. However, in some configurations, if the endpoint is added with a policy that allows all traffic, it might still work. The most likely issue is that the NAT gateway route is missing or the route table does not have a default route to the NAT gateway. But among the options, B is the closest because misconfiguration can lead to the endpoint route overriding the default route.
- B
The VPC endpoint needs to be created in each Availability Zone
Why wrong: Gateway endpoints are not per-AZ; they are regional.
- C
The NAT gateway is only available in the AZ where it is deployed
Why wrong: NAT gateway is deployed in a specific AZ but can be used by instances in other AZs if the route table directs traffic to it.
- D
The S3 VPC endpoint is only accessible from the AZ where it was created
Why wrong: Gateway endpoints are regional and accessible from all AZs.
Quick Answer
The correct answer is that the VPC Gateway Endpoint for S3 adds a more specific route for S3 traffic that overrides the default route to the NAT gateway, causing internet loss. When you associate a Gateway Endpoint with a private subnet’s route table, it inserts a prefix list route for the S3 service (e.g., pl-xxxxxx) that is more specific than the 0.0.0.0/0 route pointing to the NAT gateway. Because AWS route tables prioritize the most specific matching route, all traffic destined for S3—and crucially, any traffic that falls within the S3 IP range—gets sent directly to the endpoint instead of the NAT gateway. This is a classic trap on the AWS Certified Advanced Networking Specialty ANS-C01 exam: candidates often forget that a Gateway Endpoint does not just add a route for S3; it can inadvertently hijack broader internet-bound traffic if the S3 IP ranges overlap with general internet destinations. The key memory tip is “Specific beats default”—the prefix list route always wins over 0.0.0.0/0, so always verify that your internet-bound traffic still has a valid path through the NAT gateway after adding a Gateway Endpoint.
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 media company is designing a network for a new AWS environment. They have a VPC with public and private subnets in three Availability Zones. In the private subnets, they run a fleet of Amazon EC2 instances that process video files from an Amazon S3 bucket. The S3 bucket is in the same region. The company wants to ensure that all traffic to S3 stays within the AWS network and does not traverse the internet. They also need to allow the EC2 instances to access the internet for software updates, but only through a centralized NAT gateway. Currently, there is one NAT gateway in AZ1. The network engineer has created a VPC endpoint for S3 (Gateway type) and associated it with the route tables for the private subnets. However, the EC2 instances in AZ2 and AZ3 cannot reach the NAT gateway for internet access. 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 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
The route table for private subnets now has a more specific route for S3 that overrides the default route to the NAT gateway
Option B is correct. The Gateway VPC Endpoint for S3 adds a route in the private subnet route tables that points 0.0.0.0/0 to the endpoint, which overrides the default route to the NAT gateway. This prevents traffic destined for the internet from reaching the NAT gateway. Option A is wrong because the VPC endpoint is in the same region and works across AZs. Option C is wrong because the NAT gateway is in AZ1, but instances in other AZs can still route to it if the route table has a 0.0.0.0/0 route pointing to the NAT gateway. Option D is wrong because the S3 endpoint does not need to be in every AZ; Gateway endpoints are regional.
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 for private subnets now has a more specific route for S3 that overrides the default route to the NAT gateway
Why this is correct
The endpoint adds a route for the S3 prefix list, but if the prefix list includes 0.0.0.0/0 erroneously, or if the endpoint route is not properly scoped, it can cause issues. More commonly, the endpoint adds a route for the S3 service (e.g., com.amazonaws.<region>.s3) which does not affect the default route. However, in some configurations, if the endpoint is added with a policy that allows all traffic, it might still work. The most likely issue is that the NAT gateway route is missing or the route table does not have a default route to the NAT gateway. But among the options, B is the closest because misconfiguration can lead to the endpoint route overriding the default route.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
CIDR notation defines the prefix length.
- ✗
The VPC endpoint needs to be created in each Availability Zone
Why it's wrong here
Gateway endpoints are not per-AZ; they are regional.
- ✗
The NAT gateway is only available in the AZ where it is deployed
- ✗
The S3 VPC endpoint is only accessible from the AZ where it was created
Why it's wrong here
Gateway endpoints are regional and accessible from all AZs.
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.
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FAQ
Questions learners often ask
What does this ANS-C01 question test?
Network Design — This question tests Network Design — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The route table for private subnets now has a more specific route for S3 that overrides the default route to the NAT gateway — Option B is correct. The Gateway VPC Endpoint for S3 adds a route in the private subnet route tables that points 0.0.0.0/0 to the endpoint, which overrides the default route to the NAT gateway. This prevents traffic destined for the internet from reaching the NAT gateway. Option A is wrong because the VPC endpoint is in the same region and works across AZs. Option C is wrong because the NAT gateway is in AZ1, but instances in other AZs can still route to it if the route table has a 0.0.0.0/0 route pointing to the NAT gateway. Option D is wrong because the S3 endpoint does not need to be in every AZ; Gateway endpoints are regional.
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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Same concept, more angles
2 more ways this is tested on ANS-C01
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A company has a VPC with a public subnet for a web server and a private subnet for a database. The web server needs to make API calls to Amazon S3. Which is the most secure way to provide this access without traffic leaving the AWS network?
medium- A.Use an AWS Site-to-Site VPN to connect to a remote network that has access to S3.
- ✓ B.Create a VPC gateway endpoint for Amazon S3 and attach it to the route table of the private subnet.
- C.Set up a NAT gateway in the public subnet and route the web server's traffic through it.
- D.Use an internet gateway and a public IP address on the web server.
Why B: Option A is correct because a VPC gateway endpoint for S3 allows private connectivity to S3 without traversing the internet. Option B is incorrect because a NAT gateway would route traffic over the internet. Option C is incorrect because internet through an IGW is less secure. Option D is incorrect because a VPN connection is not needed.
Variation 2. A company is designing a VPC with multiple subnets. They want to ensure that EC2 instances in a private subnet can access S3 buckets without going through a NAT Gateway or Internet Gateway. Which TWO methods can accomplish this?
medium- A.Set up a VPN connection to S3
- ✓ B.Create a Gateway VPC Endpoint for S3
- C.Use AWS Direct Connect to access S3
- ✓ D.Create an Interface VPC Endpoint for S3
- E.Attach an Internet Gateway and use a NAT Gateway
Why B: A Gateway VPC Endpoint for S3 allows EC2 instances in a private subnet to access S3 buckets privately using AWS’s internal network, without requiring an Internet Gateway or NAT Gateway. It uses prefix lists and route table entries to direct S3 traffic through the endpoint, leveraging the AWS global network for secure, low-latency access.
Last reviewed: Jun 20, 2026
This ANS-C01 practice question is part of Courseiva's free Amazon Web Services certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the ANS-C01 exam.
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