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 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?
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 to the on-premises CIDR via the Transit Gateway
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.
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 group of the instances does not allow inbound traffic from on-premises
Why it's wrong here
Security groups can block but routing is the more likely issue.
✗
The Direct Connect virtual interface is not associated with the Transit Gateway
Why it's wrong here
Association may be correct, but route propagation is missing.
✓
The VPC route table does not have a route to the on-premises CIDR via the Transit Gateway
Why this is correct
A specific route for 10.0.0.0/8 via Transit Gateway must exist.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The on-premises router is not advertising the 10.0.0.0/8 prefix to AWS
Why it's wrong here
On-premises advertises the prefix; the issue is in the VPC route table.
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: The VPC route table does not have a route to the on-premises CIDR via the Transit Gateway — 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.
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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