The answer is that the VPC route table lacks a route pointing to the Transit Gateway for on-premises traffic. This is because the VPC has a local route for its own CIDR 10.0.0.0/16, but the on-premises network uses the overlapping CIDR 10.0.0.0/8, so return traffic from the VPC back to on-premises has no explicit path to the Transit Gateway—it simply stays local. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of route table precedence and Transit Gateway routing, where a more specific static route in the VPC route table is required to override the local route for overlapping address space. A common trap is assuming the Transit Gateway route table alone handles all routing, but the VPC must explicitly direct return traffic to the TGW attachment. Memory tip: think "local locks, TGW unlocks"—the VPC’s local route blocks overlapping traffic unless you add a specific route to the Transit Gateway.
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 company uses AWS Transit Gateway to connect multiple VPCs and on-premises networks. They have a VPC with a CIDR of 10.0.0.0/16 and an on-premises network with CIDR 10.0.0.0/8. The Transit Gateway route table has a static route for 10.0.0.0/8 pointing to the VPN attachment. However, traffic from on-premises to the VPC is not working. 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 VPC route table does not have a route pointing to the Transit Gateway for on-premises traffic.
Option C is correct because the VPC's route table has a local route for 10.0.0.0/16, but a more specific route to the Transit Gateway is needed for return traffic. Option A is incorrect because the TGW route table is correct. Option B is incorrect because overlapping CIDRs cause issues, but the TGW can handle it with more specific routes. Option D is incorrect because the VPN attachment is used.
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 VPN attachment is in the wrong TGW route table.
Why it's wrong here
Attachment is associated with the correct route table.
✓
The VPC route table does not have a route pointing to the Transit Gateway for on-premises traffic.
Why this is correct
Return traffic needs a route to TGW.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Overlap is problematic but not the direct cause here.
✗
The Transit Gateway route table does not have a route for the VPC CIDR.
Why it's wrong here
TGW has a route for 10.0.0.0/8, which covers the VPC.
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 pointing to the Transit Gateway for on-premises traffic. — Option C is correct because the VPC's route table has a local route for 10.0.0.0/16, but a more specific route to the Transit Gateway is needed for return traffic. Option A is incorrect because the TGW route table is correct. Option B is incorrect because overlapping CIDRs cause issues, but the TGW can handle it with more specific routes. Option D is incorrect because the VPN attachment is used.
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 is using AWS Transit Gateway to connect multiple VPCs and an on-premises network via AWS Direct Connect. The on-premises network advertises the 10.0.0.0/8 prefix. One VPC has a route to the Transit Gateway for 0.0.0.0/0. Instances in that VPC can reach the internet via a NAT gateway but cannot reach on-premises resources. What is the most likely issue?
medium
A.The security group of the instances does not allow inbound traffic from on-premises
B.The Direct Connect virtual interface is not associated with the Transit Gateway
✓ C.The VPC route table does not have a route to the on-premises CIDR via the Transit Gateway
D.The on-premises router is not advertising the 10.0.0.0/8 prefix to AWS
Why C: Option D is correct because the more specific 10.0.0.0/8 route from on-premises should be preferred over the default route. If propagation is not enabled, the route is missing. Option A is wrong because the NAT gateway is for internet access, not on-premises. Option B is wrong because security groups can block traffic but the question implies connectivity issue due to routing. Option C is wrong because Direct Connect bandwidth does not affect routing.
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This ANS-C01 practice question is part of Courseiva's free Amazon Web Services certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the ANS-C01 exam.
Question Discussion
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