The correct answer is to check the route table associated with the EC2 instance's subnet for a route to the on-premises network. When troubleshooting a Site-to-Site VPN where the tunnel is UP but ping fails due to a missing route, the tunnel status confirms that the IPsec negotiation and encryption are working, so the issue is almost certainly at the network layer—specifically, the VPC subnet’s route table lacks a destination route pointing to the on-premises CIDR block via the virtual private gateway. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your ability to isolate Layer 3 routing problems from Layer 2 or Layer 4 issues; a common trap is to immediately suspect the tunnel configuration or pre-shared keys when the tunnel is already up. Remember, a green tunnel light only means the VPN is encrypted and authenticated—it does not guarantee that packets have a path to leave the subnet. Memory tip: “Tunnel up, no ping? Check the route, not the key.”
ANS-C01 Network Implementation Practice Question
This ANS-C01 practice question tests your understanding of network implementation. 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 network engineer is troubleshooting connectivity between an EC2 instance in a VPC and an on-premises server connected via AWS Site-to-Site VPN. The ping from the EC2 instance to the on-premises server fails. The VPN tunnel status shows 'UP'. Which configuration should the engineer check first?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "first"
Why it matters: Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Check the route table associated with the EC2 instance's subnet for a route to the on-premises network
Option C is correct because the VPN is up, so the most likely issue is that the route table for the subnet containing the EC2 instance does not have a route to the on-premises network via the virtual private gateway. Option A is wrong because the tunnel is up. Option B is wrong because the VPN is up, so the pre-shared key is correct. Option D is wrong because the customer gateway device is on-premises, not in AWS.
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.
✗
Ensure the customer gateway device is configured with the correct public IP address
Why it's wrong here
The customer gateway is on-premises; the tunnel being up indicates the public IP is correct.
✗
Check that the pre-shared key matches on both sides
Why it's wrong here
If the pre-shared key didn't match, the tunnel would not be up.
✓
Check the route table associated with the EC2 instance's subnet for a route to the on-premises network
Why this is correct
Without a route pointing traffic to the virtual private gateway, the VPN cannot forward packets.
Clue confirmation
The clue word "first" in the question point toward this answer.
Verify that the VPN tunnel is using the correct encryption algorithms
Why it's wrong here
The tunnel is up, so encryption settings are correct.
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 Implementation — This question tests Network Implementation — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: Check the route table associated with the EC2 instance's subnet for a route to the on-premises network — Option C is correct because the VPN is up, so the most likely issue is that the route table for the subnet containing the EC2 instance does not have a route to the on-premises network via the virtual private gateway. Option A is wrong because the tunnel is up. Option B is wrong because the VPN is up, so the pre-shared key is correct. Option D is wrong because the customer gateway device is on-premises, not in AWS.
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: "first". Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
What is the key concept behind this question?
CIDR notation defines the prefix length.
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