The answer is that the route table for the NAT Gateway's subnet lacks a route to the Internet Gateway. This is correct because a NAT Gateway, despite being placed in a public subnet, cannot forward traffic to the internet unless its own subnet has a default route (0.0.0.0/0) pointing to an Internet Gateway (IGW). The NAT Gateway acts as a bridge: it receives traffic from private instances and must send it out through the IGW, but if the public subnet’s route table does not direct that outbound traffic to the IGW, the NAT Gateway has no path to the internet. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of VPC routing layers and the dependency between NAT Gateway placement and IGW connectivity. A common trap is to focus only on the private subnet’s route table while overlooking the public subnet’s configuration. Memory tip: “NAT needs a path to IGW—check the public subnet’s route table first.”
ANS-C01 Network Design Practice Question
This ANS-C01 practice question tests your understanding of network design. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
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?
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.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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
NATGateway needs a route to IGW for internet access.
Related concept
Read the scenario before looking for a memorised answer.
Common exam trap: answer the scenario, not the keyword
AWS often tests the misconception that configuring the private subnet's route table to point to the NATGateway is sufficient, while ignoring that the NAT Gateway's own subnet must have a route to the Internet Gateway for outbound connectivity.
Detailed technical explanation
How to think about this question
The NATGateway's Elastic Network Interface (ENI) must be in a public subnet with a route table entry of 0.0.0.0/0 → IGW to enable translation and forwarding of traffic. The NAT Gateway performs source network address translation (SNAT), replacing private instance source IPs with its own Elastic IP, and relies on the IGW route for return traffic. Without this route, the NAT Gateway can receive packets from private instances but cannot send them to the internet, causing a silent failure.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A healthcare organisation deploys an application with a public-facing web tier and a private database tier. The database subnet has no public IP and only accepts connections from the web tier's security group. Questions like this test whether you can design cloud network isolation using VNets/VPCs, subnets, and security group rules.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Network Design — This question tests Network Design — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: 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.
What should I do if I get this ANS-C01 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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