ANS-C01 Network Implementation Practice Question
A company has a VPC with a public subnet and a private subnet. An EC2 instance in the public subnet is configured as a NAT instance. The company wants to replace the NAT instance with a NAT gateway for better availability and maintenance. After creating a NAT gateway in the public subnet and updating the route table of the private subnet, traffic from the private subnet cannot reach the internet. What is the MOST likely cause?
⚠ Common exam trap
Many candidates assume a NAT gateway can function without an Elastic IP, similar to a NAT instance, or they may incorrectly focus on route table misconfigurations when the route table was already updated.
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 NAT gateway does not have an Elastic IP address associated with it.
A NAT gateway requires an Elastic IP address to function as a gateway for outbound internet traffic. Without an Elastic IP, the NAT gateway cannot perform source network address translation (SNAT), so instances in the private subnet cannot reach the internet. This is the most likely cause given that the route table update was already performed.
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 still points to the NAT instance for internet traffic.
Why it's wrong here
The public subnet's route table does not affect private subnet traffic.
- ✗
The security group attached to the NAT gateway is blocking outbound traffic.
Why it's wrong here
NAT gateways do not have security groups; they use network ACLs.
- ✗
The private subnet's route table has a route for 0.0.0.0/0 pointing to the NAT instance instead of the NAT gateway.
Why it's wrong here
The scenario says they updated the route table to point to the NAT gateway.
- ✓
The NAT gateway does not have an Elastic IP address associated with it.
Why this is correct
A NAT gateway requires an Elastic IP for outbound traffic.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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