The answer is that the private subnet route table lacks a specific route to the on-premises network via the virtual private gateway. When a private subnet cannot reach on-premises VPN after NAT gateway, the root cause is almost always a missing or misdirected route: the default route (0.0.0.0/0) points to the NAT Gateway for internet access, but traffic destined for the on-premises CIDR must be explicitly routed to the Virtual Private Gateway (VGW). Without that specific route, packets intended for the on-premises network are sent to the NAT Gateway, which has no path to the VPN and silently drops them. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of route table priority and the distinction between internet-bound and VPN-bound traffic. A common trap is assuming the NAT Gateway can handle all outbound traffic, but it only works for internet destinations—never for VPN-connected networks. Memory tip: think of the NAT Gateway as your “internet door” and the VGW as your “office door”; you need a separate sign (route) pointing to each.
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's VPC has an internet gateway and a NAT Gateway in a public subnet. The private subnet route table has a default route pointing to the NAT Gateway. EC2 instances in the private subnet can access the internet, but cannot access an on-premises network connected via AWS Site-to-Site VPN. What is the most likely cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The private subnet route table does not have a route to the on-premises network via the virtual private gateway.
The private subnet route table has a default route (0.0.0.0/0) pointing to the NAT Gateway, which allows outbound internet traffic. However, traffic destined for the on-premises network must be routed via the Virtual Private Gateway (VGW) attached to the VPC. Since the route table lacks a specific route (e.g., 10.0.0.0/8) pointing to the VGW, packets to the on-premises CIDR are instead sent to the NAT Gateway, which drops them because it has no route or interface for the on-premises network.
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 private subnet route table does not have a route to the on-premises network via the virtual private gateway.
The VPN connection is not advertising the on-premises CIDR via BGP.
Why it's wrong here
BGP would advertise the route, but the route table lacks a specific route.
Common exam traps
Common exam trap: answer the scenario, not the keyword
AWS often tests the misconception that a NATGateway can forward traffic to a VPN or that the internet gateway is responsible for VPN routing, when in fact the route table's destination-based forwarding is the sole determinant of where traffic goes.
Detailed technical explanation
How to think about this question
In AWS, a Virtual Private Gateway (VGW) is the VPN concentrator on the VPC side, and traffic destined for the on-premises network must be explicitly routed to the VGW via a route entry (either static or propagated from BGP). The NAT Gateway only handles traffic matching the 0.0.0.0/0 route; any traffic with a destination that matches a more specific route (like the on-premises CIDR) will follow that route. A common real-world scenario is when an organization uses both a NAT Gateway for internet access and a VPN for hybrid connectivity, requiring careful route table design to avoid asymmetric routing or blackholing.
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 company's IT admin needs to give a contractor read-only access to production logs without sharing account credentials. Using role-based access control (RBAC) and temporary scoped permissions — not a permanent shared password — is the correct pattern. Questions like this test whether you can apply least-privilege access across cloud identity services.
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 private subnet route table does not have a route to the on-premises network via the virtual private gateway. — The private subnet route table has a default route (0.0.0.0/0) pointing to the NAT Gateway, which allows outbound internet traffic. However, traffic destined for the on-premises network must be routed via the Virtual Private Gateway (VGW) attached to the VPC. Since the route table lacks a specific route (e.g., 10.0.0.0/8) pointing to the VGW, packets to the on-premises CIDR are instead sent to the NAT Gateway, which drops them because it has no route or interface for the on-premises network.
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.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
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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