The answer is a missing route in the subnet route table pointing to the virtual private gateway. When a Site-to-Site VPN tunnel is up but EC2 instances in the VPC are unreachable from on-premises, the most likely cause is that the subnet’s route table lacks a route for the on-premises CIDR block targeting the virtual private gateway (VGW). Without this route, return traffic from the VPC has no path back to the on-premises network, even though the VPN tunnel itself is established. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of VPC routing fundamentals versus common misdirection like security group rules or customer gateway static routes. A frequent trap is assuming the tunnel status guarantees connectivity, but the VPC subnet must explicitly direct traffic to the VGW. Remember the memory tip: “Tunnel up, traffic stuck? Check the route table for the VGW buck.”
ANS-C01 Network Security, Compliance and Governance Practice Question
This ANS-C01 practice question tests your understanding of network security, compliance and governance. 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 uses AWS Site-to-Site VPN to connect its on-premises network to a VPC. The VPN tunnel is up, but traffic from on-premises cannot reach EC2 instances in the VPC. The EC2 instances have a security group that allows inbound ICMP from the on-premises CIDR. 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 subnet route table does not have a route for the on-premises CIDR pointing to the virtual private gateway.
Option B is correct because the route table of the VPC subnet must have a route pointing the on-premises CIDR to the virtual private gateway (VGW). Option A is wrong because the VPN static routes are configured on the customer gateway side. Option C is wrong because the security group rule is correct. Option D is wrong because the route table association is not the issue.
Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
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 rule is using the wrong protocol.
The subnet is not associated with the main route table.
Why it's wrong here
Association is fine; the route is missing.
Common exam traps
Common exam trap: usable hosts are not the same as total addresses
Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.
Detailed technical explanation
How to think about this question
Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.
KKey Concepts to Remember
CIDR notation defines the prefix length.
Block size helps identify subnet boundaries.
Network and broadcast addresses are not usable hosts in normal IPv4 subnets.
The required host count determines the smallest suitable subnet.
TExam Day Tips
→Write the block size before choosing the subnet.
→Check whether the question asks for hosts, subnets or a specific address range.
→Do not confuse /24, /25, /26 and /27 host counts.
Key takeaway
Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
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.
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related ANS-C01 subnetting questions on CIDR, address ranges, and subnet selection.
Network Security, Compliance and Governance — This question tests Network Security, Compliance and Governance — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The subnet route table does not have a route for the on-premises CIDR pointing to the virtual private gateway. — Option B is correct because the route table of the VPC subnet must have a route pointing the on-premises CIDR to the virtual private gateway (VGW). Option A is wrong because the VPN static routes are configured on the customer gateway side. Option C is wrong because the security group rule is correct. Option D is wrong because the route table association is not the issue.
What should I do if I get this ANS-C01 question wrong?
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related ANS-C01 subnetting questions on CIDR, address ranges, and subnet selection.
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?
CIDR notation defines the prefix length.
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Question Discussion
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