ANS-C01 Network Design Practice Question
A company has a VPC with a public subnet and a private subnet. They have a NAT Gateway in the public subnet. Instances in the private subnet need to download patches from the internet. The route table for the private subnet has a default route (0.0.0.0/0) pointing to the NAT Gateway. However, instances cannot reach the internet. What is a possible cause?
⚠ Common exam trap
AWS often tests the misconception that configuring the private subnet's route table to point to the NAT Gateway is sufficient, while ignoring that the NAT Gateway's own subnet must have a route to the Internet Gateway for outbound connectivity.
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 route table for the NAT Gateway's subnet does not have a route to an Internet Gateway.
For a NAT Gateway to route traffic to the internet, its subnet must have a route to an Internet Gateway (IGW). The NAT Gateway resides in the public subnet, and the public subnet's route table must include a default route (0.0.0.0/0) pointing to the 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.
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 route table for the NAT Gateway's subnet does not have a route to an Internet Gateway.
Why this is correct
NAT Gateway needs a route to IGW for internet access.
- ✗
The NAT Gateway is in a private subnet.
Why it's wrong here
NAT Gateway must be in a public subnet.
- ✗
The network ACL on the private subnet is blocking outbound traffic.
Why it's wrong here
Could be, but less likely than missing IGW route.
- ✗
The NAT Gateway does not have a security group allowing outbound traffic.
Why it's wrong here
NAT Gateway does not use security groups.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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