The answer is that the users in the 10.1.0.0/16 VPC are likely accessing the S3 bucket through a NAT Gateway, which changes their source IP address. The bucket policy uses the aws:SourceIp condition key, which evaluates the actual IP address seen by S3; when traffic traverses a NAT Gateway, the source IP becomes the NAT’s public IP rather than the user’s private RFC 1918 address, causing the condition to fail. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of how NAT interacts with S3 bucket policy source IP conditions—a common trap is assuming private IP ranges are preserved across VPC boundaries. Remember that aws:SourceIp only works with public IPs, so if users are behind a NAT, the policy must use aws:VpcSourceIp or a VPC endpoint condition instead. Memory tip: “NAT masks the source—SourceIp fails the course.”
ANS-C01 Network Security, Compliance and Governance Practice Question
This ANS-C01 practice question tests your understanding of network security, compliance and governance. 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 applies this bucket policy to an S3 bucket. Users in a different VPC with IP range 10.0.0.0/16 are able to access objects, but users in a different VPC with IP range 10.1.0.0/16 cannot. 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 users are accessing the bucket through a NAT Gateway, which changes the source IP
The condition 'aws:SourceIp' checks the source IP address. If the users in 10.1.0.0/16 are accessing via a NAT, the source IP will be the NAT's public IP, not the private IP, and will not match the condition. Option A is wrong because the policy allows all principals. Option B is wrong because the resource is correct. Option C is wrong because the action is correct.
Key principle: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
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 bucket policy uses the wrong condition key
Why it's wrong here
'aws:SourceIp' is correct for IP-based conditions.
✓
The users are accessing the bucket through a NAT Gateway, which changes the source IP
Why this is correct
NAT changes private IP to public IP, which does not match the condition.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Static NAT maps one inside address to one outside address.
✗
The bucket policy specifies the wrong resource ARN
Why it's wrong here
The resource ARN is correct for the bucket.
✗
The bucket policy does not specify a principal
Why it's wrong here
Principal '*' allows all, so that is not the issue.
Common exam traps
Common exam trap: NAT rules depend on direction and matching traffic
NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.
Detailed technical explanation
How to think about this question
NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.
KKey Concepts to Remember
Static NAT maps one inside address to one outside address.
PAT allows many inside hosts to share one public address using ports.
Inside local and inside global describe the private and translated addresses.
NAT ACLs identify traffic for translation, not always security filtering.
TExam Day Tips
→Identify inside and outside interfaces first.
→Check whether the scenario needs static NAT, dynamic NAT or PAT.
→Do not confuse NAT matching ACLs with normal packet-filtering intent.
Key takeaway
NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
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 the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related ANS-C01 NAT questions on configuration and troubleshooting.
Network Security, Compliance and Governance — This question tests Network Security, Compliance and Governance — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: The users are accessing the bucket through a NAT Gateway, which changes the source IP — The condition 'aws:SourceIp' checks the source IP address. If the users in 10.1.0.0/16 are accessing via a NAT, the source IP will be the NAT's public IP, not the private IP, and will not match the condition. Option A is wrong because the policy allows all principals. Option B is wrong because the resource is correct. Option C is wrong because the action is correct.
What should I do if I get this ANS-C01 question wrong?
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related ANS-C01 NAT questions on configuration and troubleshooting.
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?
Static NAT maps one inside address to one outside address.
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