ANS-C01 Network Implementation Practice Question
A company has a VPC with a public subnet and a private subnet. An EC2 instance in the private subnet needs to initiate outbound connections to the internet. The company has a NAT gateway in the public subnet. The NAT gateway has an Elastic IP. The private subnet route table has a default route pointing to the NAT gateway. However, the EC2 instance cannot reach the internet. What is the most likely cause?
⚠ Common exam trap
The ANS-C01 exam often tests the misconception that NAT gateway or internet gateway configuration is the sole cause of connectivity failures, while in reality, external firewalls or destination-side IP filtering can independently block traffic even when all AWS-side networking is correctly configured.
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's Elastic IP is not allowed by the destination's firewall.
The NAT gateway's Elastic IP is a public IP address that the destination server sees as the source of the outbound traffic. If the destination's firewall (e.g., a security appliance or an internet-based service) explicitly blocks traffic from that Elastic IP, the EC2 instance will be unable to reach the internet, even though the VPC routing and NAT configuration are correct. This is a common scenario when the destination uses IP-based allowlists or blacklists.
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 security group of the EC2 instance blocks outbound traffic.
Why it's wrong here
Security groups are stateful; outbound is allowed by default.
- ✗
The route table in the public subnet does not have a default route to the internet gateway.
Why it's wrong here
Public subnet route table needs default route to IGW for NAT gateway to work.
- ✗
The network ACL of the private subnet blocks inbound traffic from the NAT gateway.
Why it's wrong here
NACLs are stateless; but the problem is outbound.
- ✓
The NAT gateway's Elastic IP is not allowed by the destination's firewall.
Why this is correct
Destination may block the public IP.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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