ANS-C01 Network Design Practice Question
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?
⚠ Common exam trap
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.
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.
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.
Why this is correct
The public subnet must have a route to an internet gateway for the NAT gateway to reach the internet.
- ✗
The NAT gateway does not have a route back to the private subnet.
Why it's wrong here
NAT gateway automatically returns traffic to the source; no explicit route is needed.
- ✗
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.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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.