The correct answer is that Transit Gateway uses the longest prefix match, so the more specific 10.0.1.0/24 route from the VPC takes precedence over the on-premises 10.0.0.0/8 advertisement. This happens because Transit Gateway evaluates route tables based on prefix length, not attachment type or administrative distance, meaning the narrower CIDR always wins regardless of whether the source is a VPC or a VPN. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this concept tests your understanding of how Transit Gateway resolves overlapping CIDR blocks in hybrid networks, a common scenario where an on-premises network advertises a supernet that conflicts with a VPC subnet. A frequent trap is assuming that VPN routes are preferred or that Transit Gateway summarises routes, but the key is remembering that longest prefix match is the sole tiebreaker. Memory tip: think of it like a zip code—a more specific five-digit code (10.0.1.0/24) always overrides a broader three-digit code (10.0.0.0/8).
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 uses AWS Transit Gateway to connect multiple VPCs and an on-premises network via VPN. The on-premises network advertises a route for 10.0.0.0/8. One VPC has a CIDR of 10.0.1.0/24. How does Transit Gateway handle the overlapping route?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The Transit Gateway uses the longest prefix match; the VPC route 10.0.1.0/24 is more specific.
Option B is correct because Transit Gateway uses the longest prefix match, so the more specific 10.0.1.0/24 route from the VPC takes precedence. Option A is wrong because it does not summarise. Option C is wrong because there is no conflict resolution based on attachment type. Option D is wrong because it does not blackhole.
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 Transit Gateway uses the longest prefix match; the VPC route 10.0.1.0/24 is more specific.
The Transit Gateway prefers the on-premises route because it is learned via BGP.
Why it's wrong here
No preference based on source; longest prefix wins.
✗
The Transit Gateway drops traffic to 10.0.1.0/24 due to conflict.
Why it's wrong here
No blackhole, it forwards to the more specific.
✗
The Transit Gateway summarises the on-premises route to 10.0.0.0/16.
Why it's wrong here
No summarisation.
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 Transit Gateway uses the longest prefix match; the VPC route 10.0.1.0/24 is more specific. — Option B is correct because Transit Gateway uses the longest prefix match, so the more specific 10.0.1.0/24 route from the VPC takes precedence. Option A is wrong because it does not summarise. Option C is wrong because there is no conflict resolution based on attachment type. Option D is wrong because it does not blackhole.
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.
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 on-premises networks. They have a VPC with a CIDR of 10.0.0.0/16 attached to the transit gateway. They also have a Direct Connect virtual interface attached to the transit gateway. The on-premises network can reach some VPCs but not the VPC with CIDR 10.0.0.0/16. The transit gateway route table has a static route for the on-premises CIDR and a route propagation from the VPC attachment. What is the most likely issue?
hard
A.The transit gateway has reached the maximum number of attachments
✓ B.The VPC CIDR overlaps with the on-premises CIDR
C.The transit gateway route table does not have a route to the on-premises network
D.The VPC attachment is not associated with the transit gateway route table
Why B: Option A is correct because the VPC CIDR 10.0.0.0/16 might overlap with the on-premises CIDR, causing routing conflicts. Option B is wrong because the VPC attachment is attached and propagating. Option C is wrong because the transit gateway route table does have routes. Option D is wrong because there is no such limit.
Last reviewed: Jun 20, 2026
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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.
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