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.
Refer to the exhibit. A network engineer has established a VPC peering connection between VPC A (10.0.0.0/16) in account 111111111111 and VPC B (192.168.0.0/16) in account 222222222222. The peering connection status is 'active'. However, instances in VPC A cannot reach instances in VPC B. 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.
The peering connection is not in the 'active' state
Why wrong: Status shows 'active'.
Route tables in one or both VPCs do not have routes to the peer CIDR
Missing routes prevent communication.
Security groups in VPC B are blocking traffic
Why wrong: Possible but not the most likely.
The CIDR blocks overlap
Why wrong: They are different.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Route tables in one or both VPCs do not have routes to the peer CIDR
Option D is correct because the route tables in both VPCs need routes to the peer CIDR via the peering connection. Option A is incorrect because the status is active. Option B is incorrect because the CIDRs are non-overlapping. Option C is incorrect because there is no indication of security groups blocking, though they could; but the most common cause is missing 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
For each option: why learners choose it and why it is or isn't the right answer here.
The peering connection is not in the 'active' state
Why it's wrong here
Status shows 'active'.
Route tables in one or both VPCs do not have routes to the peer CIDR
Why this is correct
Missing routes prevent communication.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
CIDR notation defines the prefix length.
Security groups in VPC B are blocking traffic
Why it's wrong here
Possible but not the most likely.
The CIDR blocks overlap
Why it's wrong here
They are different.
Common exam traps
Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.
Trap categories for this question
Command / output trap
Status shows 'active'.
Detailed technical explanation
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.
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
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
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
Network Management and Operations — This question tests Network Management and Operations — CIDR notation defines the prefix length..
The correct answer is: Route tables in one or both VPCs do not have routes to the peer CIDR — Option D is correct because the route tables in both VPCs need routes to the peer CIDR via the peering connection. Option A is incorrect because the status is active. Option B is incorrect because the CIDRs are non-overlapping. Option C is incorrect because there is no indication of security groups blocking, though they could; but the most common cause is missing routes.
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.
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
CIDR notation defines the prefix length.
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Last reviewed: Jun 20, 2026
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