ANS-C01 Network Design Practice Question
A company wants to provide internet access to instances in a private subnet using a NAT gateway. The NAT gateway is deployed in a public subnet with an Elastic IP. The private subnet route table has a default route pointing to the NAT gateway. However, instances in the private subnet cannot access the internet. What is the most likely cause?
⚠ Common exam trap
AWS often tests the misconception that a NAT gateway automatically has internet access simply because it is in a public subnet, but the critical missing piece is the explicit route to the internet gateway in that 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 route table does not have a route to the internet gateway.
The NAT gateway is deployed in a 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. This is the most likely cause because the NAT gateway itself needs internet access via the IGW to translate and forward traffic.
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 route table does not have a route to the internet gateway.
Why this is correct
NAT gateway needs internet gateway route in its subnet.
- ✗
The security group assigned to the NAT gateway blocks outbound traffic.
Why it's wrong here
NAT gateway security groups are not the primary cause.
- ✗
The private subnet route table has a route to the internet gateway instead of the NAT gateway.
Why it's wrong here
Private subnet route table should point to NAT gateway, not internet gateway.
- ✗
The NAT gateway does not have an Elastic IP associated.
Why it's wrong here
Elastic IP is required and stated as present.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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