The answer is the private subnet route table does not have a route for the on-premises CIDR pointing to the VGW. This is correct because even when a Site-to-Site VPN tunnel is established between a Virtual Private Gateway and a Customer Gateway, traffic from a private subnet will not reach the on-premises network unless the subnet’s route table explicitly directs the on-premises CIDR block to the VGW. Without that route, the EC2 instance’s packets have no path to the VPN tunnel, regardless of security group or network ACL settings. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of how routing tables govern traffic flow in a hybrid network—a common trap is assuming a working VPN tunnel alone guarantees connectivity. Remember the key principle: a VPN tunnel is a link, but a route is the map; both are required. Memory tip: “Tunnel up, route down? Packets drown.”
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. An EC2 instance in the private subnet needs to communicate with an on-premises server using an IPsec VPN. The company has set up a Virtual Private Gateway (VGW) and a Customer Gateway (CGW) with a Site-to-Site VPN connection. The VPN tunnel is established. However, the EC2 instance cannot ping the on-premises server. The security groups and network ACLs allow all traffic. 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 for the on-premises CIDR pointing to the VGW
Option B is correct because the private subnet route table must include a route for the on-premises CIDR pointing to the VGW. Option A is wrong because the VGW is attached and the VPN is established. Option C is wrong because the VPN is up. Option D is wrong because the VPN is established.
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 VGW is not configured with the correct BGP ASN
Why it's wrong here
The tunnel is established, so BGP configuration is likely correct.
✗
The VGW is not attached to the VPC
Why it's wrong here
The VPN tunnel is established, indicating the VGW is attached.
✗
The VPN tunnel is not in the UP state
Why it's wrong here
The problem states the tunnel is established.
✓
The private subnet route table does not have a route for the on-premises CIDR pointing to the VGW
Why this is correct
Without this route, traffic from the instance to on-premises is dropped.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
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 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.
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 Design — This question tests Network Design — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The private subnet route table does not have a route for the on-premises CIDR pointing to the VGW — Option B is correct because the private subnet route table must include a route for the on-premises CIDR pointing to the VGW. Option A is wrong because the VGW is attached and the VPN is established. Option C is wrong because the VPN is up. Option D is wrong because the VPN is established.
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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