- A
The public subnet's route table does not have a route to an internet gateway.
The public subnet must have a route to an internet gateway for the NAT gateway to reach the internet.
- B
The NAT gateway does not have a route back to the private subnet.
Why wrong: NAT gateway automatically returns traffic to the source; no explicit route is needed.
- C
The NAT gateway is not associated with a security group that allows inbound internet traffic.
Why wrong: NAT gateways do not use security groups.
- D
The security group attached to the EC2 instances does not allow outbound traffic to the internet.
Why wrong: Security groups are stateful; outbound traffic is allowed by default unless explicitly denied.
Question 1,517 of 1,705
NAT Gateway Troubleshooting: Missing Internet Gateway Route
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 company has a VPC with a public subnet containing a NAT gateway and a private subnet containing Amazon EC2 instances. The instances in the private subnet need to download patches from the internet. The NAT gateway is in the public subnet and has an Elastic IP address. The private subnet's route table has a default route pointing to the NAT gateway. However, the instances cannot reach the internet. 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 public subnet's route table does not have a route to an internet gateway.
The NAT gateway resides in the public subnet, but for it to route traffic to the internet, the public subnet's route table must have a default route (0.0.0.0/0) pointing to an internet gateway (IGW). Without this route, the NAT gateway cannot forward traffic from the private subnet to the internet, even though the private subnet's route table correctly points to the NAT gateway. The missing IGW route in the public subnet breaks the outbound path.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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 public subnet's route table does not have a route to an internet gateway.
- ✗
The NAT gateway does not have a route back to the private subnet.
- ✗
The NAT gateway is not associated with a security group that allows inbound internet traffic.
Why it's wrong here
NAT gateways do not use security groups.
- ✗
The security group attached to the EC2 instances does not allow outbound traffic to the internet.
Why it's wrong here
Security groups are stateful; outbound traffic is allowed by default unless explicitly denied.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The ANS-C01 exam often tests the misconception that a NAT gateway automatically has internet access if it has an Elastic IP, but the Elastic IP alone does not provide a path to the internet without a route to an internet gateway in the subnet's route table.
Detailed technical explanation
How to think about this question
A NAT gateway is an AWS-managed service that performs source network address translation (SNAT), replacing the private IP of the instance with the NAT gateway's Elastic IP. For this to work, the NAT gateway must be in a public subnet with a route to an IGW, because the NAT gateway itself sends packets to the internet via the IGW. Without that route, the NAT gateway's traffic is dropped, as it has no path to the internet. This is a common misconfiguration where the public subnet's route table is overlooked.
KKey Concepts to Remember
- Read the scenario before looking for a memorised answer.
- Find the constraint that changes the correct option.
- Eliminate answers that are true in general but not in this case.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A company's IT admin needs to give a contractor read-only access to production logs without sharing account credentials. Using role-based access control (RBAC) and temporary scoped permissions — not a permanent shared password — is the correct pattern. Questions like this test whether you can apply least-privilege access across cloud identity services.
Visual reference
What to study next
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FAQ
Questions learners often ask
What does this ANS-C01 question test?
Network Design — This question tests Network Design — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The public subnet's route table does not have a route to an internet gateway. — The NAT gateway resides in the public subnet, but for it to route traffic to the internet, the public subnet's route table must have a default route (0.0.0.0/0) pointing to an internet gateway (IGW). Without this route, the NAT gateway cannot forward traffic from the private subnet to the internet, even though the private subnet's route table correctly points to the NAT gateway. The missing IGW route in the public subnet breaks the outbound path.
What should I do if I get this ANS-C01 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jul 4, 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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