The answer is overlapping CIDR blocks. VPC peering fails with overlapping CIDR blocks because the route tables in each VPC cannot distinguish between traffic destined for the local VPC and the peered VPC when both use the same IP range, such as 10.0.0.0/16. Since AWS routes traffic based on the most specific prefix, identical CIDRs create an ambiguity where packets intended for the remote VPC are instead routed locally, breaking connectivity entirely. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this concept tests your understanding of VPC peering limitations and is a common trap where candidates assume a peering connection alone guarantees reachability. A key memory tip: if two VPCs share the same CIDR, think of them as identical twins—the network simply cannot tell them apart, so peering will never work.
ANS-C01 Network Implementation Practice Question
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 troubleshooting connectivity between two VPCs (VPC-A and VPC-B) that are peered together. Both VPCs are in the same region. VPC-A has a CIDR of 10.0.0.0/16 and VPC-B has a CIDR of 10.0.0.0/16. The peering connection is established and the route tables are updated. However, EC2 instances in VPC-A cannot ping EC2 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.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The VPCs have overlapping CIDR blocks.
Option C is correct. VPC peering does not support overlapping CIDR blocks. The routes cannot distinguish between the two VPCs. Option A is wrong because security groups can allow ICMP. Option B is wrong because private IP addresses are reachable if routes exist. Option D is wrong because same region peering works.
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 route tables do not have routes to the private IP addresses of the instances.
Why it's wrong here
Routes to the CIDR are present, but overlapping CIDRs cause ambiguity.
✓
The VPCs have overlapping CIDR blocks.
Why this is correct
Overlapping CIDRs prevent proper routing in VPC peering.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The security groups in VPC-B do not allow inbound ICMP traffic from VPC-A.
Why it's wrong here
While true, the primary issue is overlapping CIDRs; even with open security groups, routing would fail.
✗
The VPCs are in different regions and peering does not work across regions.
Why it's wrong here
The question states same region.
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 VPCs have overlapping CIDR blocks. — Option C is correct. VPC peering does not support overlapping CIDR blocks. The routes cannot distinguish between the two VPCs. Option A is wrong because security groups can allow ICMP. Option B is wrong because private IP addresses are reachable if routes exist. Option D is wrong because same region peering works.
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.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
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.
Question Discussion
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
Sign in to join the discussion.