The answer is that the EC2 instance is accessing the S3 bucket through a NAT Gateway, so the source IP is the public IP of the NAT Gateway, which does not match the condition. The aws:SourceIp condition in an S3 bucket policy evaluates the IP address of the requester, and when an instance routes traffic through a NAT Gateway to reach the internet, the source IP becomes the NAT Gateway’s public IP rather than the instance’s private IP. This is a critical distinction on the AWS Certified Advanced Networking Specialty ANS-C01 exam, which tests your understanding of how network paths affect policy evaluation—specifically, that VPC endpoints preserve private IPs, while internet-bound traffic via a NAT Gateway or internet gateway exposes a public IP that likely falls outside the allowed CIDR. A common trap is assuming the console always uses the instance’s IP, but the console traffic goes over the internet, not through a VPC endpoint. Memory tip: “NAT masks the match”—if traffic goes through a NAT, the source IP condition won’t match your VPC CIDR.
ANS-C01 Network Implementation Practice Question
This ANS-C01 practice question tests your understanding of network implementation. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. 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.
Refer to the exhibit. A company has an S3 bucket with the bucket policy shown. An EC2 instance in a VPC with CIDR 10.0.0.0/16 tries to retrieve an object from the bucket using the S3 console, but receives an 'Access Denied' error. The instance's security group allows all outbound traffic. 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.
The EC2 instance is accessing the S3 bucket through a NAT Gateway, so the source IP is the public IP of the NAT Gateway, which does not match the condition.
The IP condition requires the source IP to be within the VPC CIDR, but via NAT the source IP is the NAT's public IP.
B
The bucket policy does not allow the s3:GetObject action.
Why wrong: The policy explicitly allows s3:GetObject.
C
The bucket policy does not specify a principal, so it defaults to deny.
Why wrong: Principal: '*' allows all principals.
D
The condition aws:SourceIp is too restrictive and blocks all traffic.
Why wrong: The condition is valid; the issue is the IP mismatch.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The EC2 instance is accessing the S3 bucket through a NAT Gateway, so the source IP is the public IP of the NAT Gateway, which does not match the condition.
The condition aws:SourceIp evaluates the IP address of the requester. For EC2 instances accessing S3 via a VPC endpoint, the source IP is the private IP, which matches 10.0.0.0/16. However, accessing S3 via the console uses the public IP of the NATGateway or internet gateway, which is not in the 10.0.0.0/16 range. Option B is wrong because the policy allows GetObject. Option C is wrong because the policy allows all principals. Option D is wrong because the bucket policy is not too restrictive; it just requires matching IP.
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 EC2 instance is accessing the S3 bucket through a NAT Gateway, so the source IP is the public IP of the NAT Gateway, which does not match the condition.
Why this is correct
The IP condition requires the source IP to be within the VPC CIDR, but via NAT the source IP is the NAT's public IP.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The bucket policy does not allow the s3:GetObject action.
Why it's wrong here
The policy explicitly allows s3:GetObject.
✗
The bucket policy does not specify a principal, so it defaults to deny.
Why it's wrong here
Principal: '*' allows all principals.
✗
The condition aws:SourceIp is too restrictive and blocks all traffic.
Why it's wrong here
The condition is valid; the issue is the IP mismatch.
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 EC2 instance is accessing the S3 bucket through a NAT Gateway, so the source IP is the public IP of the NAT Gateway, which does not match the condition. — The condition aws:SourceIp evaluates the IP address of the requester. For EC2 instances accessing S3 via a VPC endpoint, the source IP is the private IP, which matches 10.0.0.0/16. However, accessing S3 via the console uses the public IP of the NAT Gateway or internet gateway, which is not in the 10.0.0.0/16 range. Option B is wrong because the policy allows GetObject. Option C is wrong because the policy allows all principals. Option D is wrong because the bucket policy is not too restrictive; it just requires matching IP.
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.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
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.
Question Discussion
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
Sign in to join the discussion.