The answer is that the private subnet's route table lacks a route for the on-premises CIDR (10.0.0.0/8) pointing to the virtual private gateway, which breaks return traffic. This occurs because Direct Connect private subnet routing requires explicit routes in the subnet’s route table for any on-premises destinations; without that route, the EC2 instance in the private subnet cannot send reply traffic back through the virtual private gateway, even though the on-premises side can initiate the ping. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of how subnet route tables override the VPC’s main route table—a common trap where candidates assume the main table’s route applies to all subnets. The NAT gateway is reachable because it resides in the public subnet, which likely inherits the main table’s route, but the private subnet’s custom table only has a 0.0.0.0/0 route to the NAT gateway, ignoring the on-premises path. Memory tip: “Private subnet, private route—if it’s not in the table, the traffic is unable.”
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 Direct Connect connection with a private VIF to a VPC in us-east-1. The VPC has two subnets: a public subnet and a private subnet. The public subnet has an internet gateway attached. The private subnet has a NAT gateway. The company's on-premises network uses the 10.0.0.0/8 IP range. The VPC CIDR is 10.1.0.0/16. The on-premises router is advertising 10.1.0.0/16 over BGP to the Direct Connect router. The company needs EC2 instances in the private subnet to initiate outbound connections to the internet for updates. The NAT gateway is in the public subnet. The route table for the private subnet has a default route (0.0.0.0/0) pointing to the NAT gateway. However, the on-premises network team reports that they can ping the private IP of the NAT gateway (10.1.0.10) but not the private IP of an EC2 instance in the private subnet (10.1.1.50). The EC2 instance's security group allows ICMP from the on-premises IP range. The VPC's main route table has a route for 10.0.0.0/8 pointing to the virtual private gateway. The VPC is attached to a virtual private gateway. 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.
The virtual private gateway is not attached to the VPC.
Why wrong: The VPC is attached to a virtual private gateway, as stated.
B
The NAT gateway is not reachable from the on-premises network.
Why wrong: The on-premises network can ping the NAT gateway, so it is reachable.
C
The security group on the EC2 instance is blocking ICMP from the on-premises IP range.
Why wrong: The security group allows ICMP from the on-premises IP range, as stated.
D
The private subnet's route table does not have a route for the on-premises CIDR (10.0.0.0/8) pointing to the virtual private gateway.
Without a route for the on-premises CIDR in the private subnet's route table, traffic from on-premises to the EC2 instance is not forwarded to the VGW.
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's route table does not have a route for the on-premises CIDR (10.0.0.0/8) pointing to the virtual private gateway.
The on-premises network can ping the NAT gateway because the NAT gateway is in the public subnet, and the route table for the public subnet likely has a route to the virtual private gateway for the on-premises CIDR. However, the EC2 instance is in the private subnet, and the private subnet's route table does not have a route for the on-premises CIDR (10.0.0.0/8) pointing to the virtual private gateway. The VPC's main route table has such a route, but the private subnet is not using the main route table; it likely has a custom route table that only has the default route to the NAT gateway. Therefore, traffic from on-premises to the EC2 instance's private IP is not routed to the virtual private gateway. Option D is correct. Option A is incorrect because the security group allows ICMP. Option B is incorrect because the NAT gateway is reachable. Option C is incorrect because the virtual private gateway is attached.
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 virtual private gateway is not attached to the VPC.
Why it's wrong here
The VPC is attached to a virtual private gateway, as stated.
✗
The NAT gateway is not reachable from the on-premises network.
Why it's wrong here
The on-premises network can ping the NATgateway, so it is reachable.
✗
The security group on the EC2 instance is blocking ICMP from the on-premises IP range.
Why it's wrong here
The security group allows ICMP from the on-premises IP range, as stated.
✓
The private subnet's route table does not have a route for the on-premises CIDR (10.0.0.0/8) pointing to the virtual private gateway.
Why this is correct
Without a route for the on-premises CIDR in the private subnet's route table, traffic from on-premises to the EC2 instance is not forwarded to the VGW.
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 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 private subnet's route table does not have a route for the on-premises CIDR (10.0.0.0/8) pointing to the virtual private gateway. — The on-premises network can ping the NAT gateway because the NAT gateway is in the public subnet, and the route table for the public subnet likely has a route to the virtual private gateway for the on-premises CIDR. However, the EC2 instance is in the private subnet, and the private subnet's route table does not have a route for the on-premises CIDR (10.0.0.0/8) pointing to the virtual private gateway. The VPC's main route table has such a route, but the private subnet is not using the main route table; it likely has a custom route table that only has the default route to the NAT gateway. Therefore, traffic from on-premises to the EC2 instance's private IP is not routed to the virtual private gateway. Option D is correct. Option A is incorrect because the security group allows ICMP. Option B is incorrect because the NAT gateway is reachable. Option C is incorrect because the virtual private gateway is attached.
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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