The answer is the missing VPC route for 192.168.0.0/16 pointing to the virtual private gateway. This is correct because even when the VPN tunnel is up, the VPC’s route table must explicitly direct traffic destined for the on-premises network toward the virtual private gateway; without that entry, the VPC drops the packets at the subnet level, as it has no path for the remote CIDR. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of the fundamental connectivity prerequisites for site-to-site VPN troubleshooting—specifically, that a tunnel being up does not guarantee traffic flow if routing is misconfigured. A common trap is assuming BGP will automatically propagate routes into the VPC route table, but static routes or propagated routes must be explicitly added. Memory tip: “Tunnel up, route missing? Check the VPC table for the remote CIDR pointing to the VGW.”
ANS-C01 Network Implementation Practice Question
This ANS-C01 practice question tests your understanding of network implementation. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. 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 CIDR of 10.0.0.0/16 and needs to establish a site-to-site VPN connection to an on-premises network with a CIDR of 192.168.0.0/16. The VPN tunnel is up, but traffic from the VPC to on-premises is not flowing. Which of the following 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 VPC route table does not have a route for 192.168.0.0/16 pointing to the virtual private gateway.
Option A is correct because the VPC route table must have a route pointing to the virtual private gateway for the on-premises CIDR. Without it, traffic is dropped. B, C, D are possible but less likely initial checks.
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 groups do not allow outbound traffic.
Why it's wrong here
Security groups are stateful and allow outbound by default.
✗
The NACLs are blocking outbound traffic.
Why it's wrong here
NACLs default allow outbound.
✓
The VPC route table does not have a route for 192.168.0.0/16 pointing to the virtual private gateway.
Why this is correct
Missing route prevents traffic from leaving VPC.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
VPN is region-specific; if in wrong region, tunnel wouldn't be up.
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 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 Implementation — This question tests Network Implementation — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The VPC route table does not have a route for 192.168.0.0/16 pointing to the virtual private gateway. — Option A is correct because the VPC route table must have a route pointing to the virtual private gateway for the on-premises CIDR. Without it, traffic is dropped. B, C, D are possible but less likely initial checks.
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.
About these practice questions
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These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A company has a VPC with a CIDR of 10.0.0.0/16. They create a subnet 10.0.1.0/24 and launch an EC2 instance with a private IP 10.0.1.5. The instance needs to communicate with an on-premises server at 172.16.0.10 over a VPN connection. The VPN connection uses a Virtual Private Gateway. The VPC route table has a route 172.16.0.0/16 pointing to the VPG. The instance cannot reach the on-premises server. What is the most likely cause?
hard
A.The security group attached to the instance does not allow outbound traffic to 172.16.0.10.
B.The instance's route table does not have a route to the internet gateway.
C.The network ACL for the subnet denies outbound traffic to the on-premises IP range.
✓ D.The on-premises router is not advertising the 172.16.0.0/16 route to the Virtual Private Gateway.
Why D: The VPN connection requires that the on-premises network advertise routes to the VPG via BGP. If the on-premises router is not advertising the 172.16.0.0/16 route, the VPG will not have a route to forward traffic. Option A (security group) would affect inbound, not outbound; Option C (NACL) would need to be checked but default NACL allows all; Option D (internet gateway) is not needed for VPN.
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Question Discussion
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