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 and a private subnet. An EC2 instance in the private subnet needs to download patches from the internet. The instance is behind a NAT Gateway in the public subnet. The download is failing. Which configuration should the network engineer check FIRST?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "first"
Why it matters: Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
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 associated with the private subnet has a default route (0.0.0.0/0) pointing to the NAT Gateway.
Option D is correct because for an EC2 instance in a private subnet to reach the internet via a NAT Gateway, the private subnet's route table must have a default route (0.0.0.0/0) pointing to the NAT Gateway. Without this route, traffic from the instance destined for the internet has no path to the NAT Gateway, causing the download to fail.
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 network ACL for the private subnet allows inbound HTTP/HTTPS traffic.
Why it's wrong here
Outbound traffic is needed, and NACL is secondary to routing.
✗
The EC2 instance has a public IP address assigned.
Why it's wrong here
Private instances do not need public IPs; they use NAT.
✗
The Internet Gateway is attached to the private subnet.
Why it's wrong here
IGW is attached to the VPC, not a subnet.
✓
The route table associated with the private subnet has a default route (0.0.0.0/0) pointing to the NAT Gateway.
Why this is correct
Correct: Without this route, traffic cannot reach the NATGateway.
Clue confirmation
The clue word "first" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
AWS often tests the misconception that a private instance needs a public IP or that NACLs are the primary cause of outbound internet failures, when in fact the missing default route to the NAT Gateway is the most common misconfiguration.
Detailed technical explanation
How to think about this question
The NATGateway resides in a public subnet with an Elastic IP and relies on the Internet Gateway for its own outbound connectivity. The private subnet's route table must have a 0.0.0.0/0 route pointing to the NAT Gateway's ENI; otherwise, traffic from the instance is dropped. Additionally, the NAT Gateway's security group (if used) and the private subnet's NACL must allow outbound HTTP/HTTPS traffic, but routing is the first check because without it, no packets reach the NAT.
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.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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 route table associated with the private subnet has a default route (0.0.0.0/0) pointing to the NAT Gateway. — Option D is correct because for an EC2 instance in a private subnet to reach the internet via a NAT Gateway, the private subnet's route table must have a default route (0.0.0.0/0) pointing to the NAT Gateway. Without this route, traffic from the instance destined for the internet has no path to the NAT Gateway, causing the download to fail.
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: "first". Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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