The answer is the NAT Gateway's security group blocking outbound HTTPS traffic. This is correct because while the EC2 instance’s security group and the subnet’s NACL both permit outbound HTTPS to 0.0.0.0/0, the NAT Gateway itself is a managed resource with its own security group that must explicitly allow inbound HTTPS from the private subnet and outbound HTTPS to S3’s endpoints; if that outbound rule is missing, the traffic is silently dropped at the gateway. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding that NAT Gateway security groups operate as a stateful filter for traffic passing through the gateway—a common trap is assuming only the instance’s security group or NACLs control the path, forgetting the gateway’s own rules. A useful memory tip: “NAT is a middleman—it needs its own permission to pass the parcel.”
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 public and private subnets. An EC2 instance in a private subnet needs to access an S3 bucket. The VPC has a NAT Gateway in the public subnet. The security group for the EC2 instance allows outbound HTTPS to 0.0.0.0/0. The NACL for the private subnet allows outbound HTTPS to 0.0.0.0/0 and inbound ephemeral ports from 0.0.0.0/0. The instance still cannot reach S3. 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 NAT Gateway's security group does not allow outbound HTTPS traffic.
Option B is correct because S3 requires HTTPS (443) for API calls, but the NATGateway's security group must allow inbound HTTPS from the private subnet and outbound to S3. If the NAT Gateway's security group does not allow outbound HTTPS to S3, traffic is dropped. Option A is wrong because the private subnet has a route to the NAT Gateway. Option C is wrong because S3 does not have a security group in the VPC. Option D is wrong because the NACL allows inbound ephemeral ports.
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 NAT Gateway's security group does not allow outbound HTTPS traffic.
Why this is correct
NATGateway's security group must allow outbound traffic to S3.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The NACL for the private subnet blocks inbound traffic from S3.
Why it's wrong here
NACL allows ephemeral ports inbound.
✗
The private subnet's route table does not have a route to the NAT Gateway.
Why it's wrong here
If route missing, traffic would not reach NATGateway.
✗
The S3 bucket policy denies access from the VPC.
Why it's wrong here
Bucket policy would affect requests, but the issue is connectivity.
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 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: The NAT Gateway's security group does not allow outbound HTTPS traffic. — Option B is correct because S3 requires HTTPS (443) for API calls, but the NAT Gateway's security group must allow inbound HTTPS from the private subnet and outbound to S3. If the NAT Gateway's security group does not allow outbound HTTPS to S3, traffic is dropped. Option A is wrong because the private subnet has a route to the NAT Gateway. Option C is wrong because S3 does not have a security group in the VPC. Option D is wrong because the NACL allows inbound ephemeral ports.
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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