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Question 905 of 1,705

Network Implementation →hardMultiple ChoiceObjective-mapped

Handling Overlapping CIDR Blocks with AWS Transit Gateway — AWS Advanced Networking Specialty Explained

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 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?

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.

A
The transit gateway has reached the maximum number of attachments
Why wrong: Not mentioned.
B
The VPC CIDR overlaps with the on-premises CIDR
Overlapping CIDRs cause routing issues in a transit gateway.
C
The transit gateway route table does not have a route to the on-premises network
Why wrong: It has a static route for on-premises.
D
The VPC attachment is not associated with the transit gateway route table
Why wrong: It is attached and propagating routes.

Full breakdown with real-world context →

Answer choices

Why each option matters

Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.

Correct answer & explanation

✓

The VPC CIDR overlaps with the on-premises CIDR

The most likely issue is that the VPC CIDR (10.0.0.0/16) overlaps with the on-premises CIDR. When CIDR blocks overlap, the transit gateway cannot distinguish between destinations in the VPC and on-premises, leading to routing conflicts. This explains why the on-premises network can reach other VPCs with non-overlapping CIDRs but not this VPC. The static route for the on-premises CIDR and the propagated route from the VPC attachment both exist in the transit gateway route table, but the overlap causes the transit gateway to forward traffic inconsistently or drop it. Options A, C, and D are incorrect: A) The transit gateway has no such attachment limit; C) The route table does have a route to the on-premises network via the static route; D) The VPC attachment is associated and propagating routes.

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 has reached the maximum number of attachments
Why it's wrong here
Not mentioned.
✓
The VPC CIDR overlaps with the on-premises CIDR
Why this is correct
Overlapping CIDRs cause routing issues in a transit gateway.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
CIDR notation defines the prefix length.
✗
The transit gateway route table does not have a route to the on-premises network
Why it's wrong here
It has a static route for on-premises.
✗
The VPC attachment is not associated with the transit gateway route table
Why it's wrong here
It is attached and propagating routes.

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.

Visual reference

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.

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FAQ

Questions learners often ask

What does this ANS-C01 question test?

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 CIDR overlaps with the on-premises CIDR — The most likely issue is that the VPC CIDR (10.0.0.0/16) overlaps with the on-premises CIDR. When CIDR blocks overlap, the transit gateway cannot distinguish between destinations in the VPC and on-premises, leading to routing conflicts. This explains why the on-premises network can reach other VPCs with non-overlapping CIDRs but not this VPC. The static route for the on-premises CIDR and the propagated route from the VPC attachment both exist in the transit gateway route table, but the overlap causes the transit gateway to forward traffic inconsistently or drop it. Options A, C, and D are incorrect: A) The transit gateway has no such attachment limit; C) The route table does have a route to the on-premises network via the static route; D) The VPC attachment is associated and propagating routes.

What should I do if I get this ANS-C01 question wrong?

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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Same concept, more angles

1 more ways this is tested on ANS-C01

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 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?

medium

✓ A.The Transit Gateway uses the longest prefix match; the VPC route 10.0.1.0/24 is more specific.
B.The Transit Gateway prefers the on-premises route because it is learned via BGP.
C.The Transit Gateway drops traffic to 10.0.1.0/24 due to conflict.
D.The Transit Gateway summarises the on-premises route to 10.0.0.0/16.

Why A: AWS Transit Gateway uses the longest prefix match (LPM) rule to select the most specific route when multiple routes overlap. In this case, the on-premises network advertises 10.0.0.0/8 via BGP, while the VPC has a directly attached CIDR of 10.0.1.0/24. Since /24 is more specific than /8, traffic destined for 10.0.1.0/24 is routed to the VPC, not the on-premises network.

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