The answer is overlapping CIDR blocks, which make VPC peering connectivity fail because AWS requires the IP address ranges of peered VPCs to be unique and non-overlapping. When both VPCs use 10.0.0.0/16, the route tables cannot distinguish which traffic belongs to which VPC, so packets are dropped or misrouted even if the peering connection status is active. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of VPC peering’s fundamental constraint—CIDR uniqueness—and is a common trap where candidates assume a peering connection status of “active” guarantees connectivity. The exam often pairs this with distractors about route propagation or transitive routing, but the core issue is that overlapping CIDRs create an unresolvable routing conflict. Memory tip: “Peering requires non-overlapping—if CIDRs clash, traffic will crash.”
ANS-C01 Network Management and Operations Practice Question
This ANS-C01 practice question tests your understanding of network management and operations. 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 has a VPC with a CIDR of 10.0.0.0/16 and needs to connect to a partner VPC with CIDR 10.0.0.0/16. Both VPCs are in the same region. They want to use VPC Peering. After creating the peering connection and adding routes, connectivity fails. 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.
Option A is correct because overlapping CIDRs are not allowed in VPC peering; routes cannot distinguish. Option B is wrong because there is no route conflict after peering. Option C is wrong because VPC peering does not support transitive routing. Option D is wrong because any status other than active would prevent connectivity, but with overlapping CIDRs, even active won't work.
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 peering connection is not set up for transitive routing.
Why it's wrong here
VPC peering is non-transitive, but that's not the issue.
✗
The peering connection status is 'pending-acceptance'.
Why it's wrong here
If pending, it wouldn't work, but overlapping is the core issue.
The route tables do not have a route to the peering connection.
Why it's wrong here
Routes are added but cannot work with overlapping CIDRs.
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 Management and Operations — This question tests Network Management and Operations — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: Overlapping CIDR blocks prevent VPC peering connectivity. — Option A is correct because overlapping CIDRs are not allowed in VPC peering; routes cannot distinguish. Option B is wrong because there is no route conflict after peering. Option C is wrong because VPC peering does not support transitive routing. Option D is wrong because any status other than active would prevent connectivity, but with overlapping CIDRs, even active won't work.
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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Question Discussion
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