The correct answer is that VPC peering requires non-overlapping CIDR blocks, so if any overlap exists, you must re-IP one VPC or use an alternative like Transit Gateway with NAT. This is because VPC peering relies on route tables that map a destination CIDR to a single target; overlapping CIDRs create routing ambiguity, as the same IP range cannot be routed to two different VPCs or on-premises networks simultaneously. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this question tests your understanding of VPC peering’s fundamental limitation with overlapping CIDRs—a common trap is assuming NAT or Transit Gateway alone can resolve the conflict, but Transit Gateway also cannot handle overlapping CIDRs without additional translation. The key memory tip is: “Peering needs clearing—no overlap, no problem.”
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 has a VPC with CIDR 10.0.0.0/16 and needs to connect to another VPC (192.168.0.0/16) using VPC peering. Both VPCs have overlapping CIDRs with some on-premises networks. What must be done to ensure proper routing?
Use AWS Transit Gateway instead and attach both VPCs.
Why wrong: Transit Gateway also does not support overlapping CIDRs for direct routing; you would need additional configurations.
B
Verify that the CIDR blocks do not overlap. If they do, you must re-IP one VPC or use other solutions like Transit Gateway with network address translation.
Without overlap, peering works. Overlap requires re-IP or alternative solutions.
C
Use a NAT gateway in one VPC to translate addresses.
Why wrong: NAT is for outbound traffic, not for solving overlapping CIDRs in peering.
D
Create the peering connection and add routes for the full CIDR ranges in both route tables.
Why wrong: Overlapping CIDRs cause routing conflicts; peering may not work.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Verify that the CIDR blocks do not overlap. If they do, you must re-IP one VPC or use other solutions like Transit Gateway with network address translation.
Option B is correct because VPC peering does not support overlapping CIDRs; you cannot have routes to the same CIDR via different targets. If there is any overlap, you cannot create the peering connection or the routes. Option A is wrong because it suggests ignoring overlap. Option C is wrong because NAT does not solve routing ambiguity. Option D is wrong because Transit Gateway also has limitations with overlapping CIDRs.
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.
✗
Use AWS Transit Gateway instead and attach both VPCs.
Why it's wrong here
Transit Gateway also does not support overlapping CIDRs for direct routing; you would need additional configurations.
✓
Verify that the CIDR blocks do not overlap. If they do, you must re-IP one VPC or use other solutions like Transit Gateway with network address translation.
Why this is correct
Without overlap, peering works. Overlap requires re-IP or alternative solutions.
Use a NAT gateway in one VPC to translate addresses.
Why it's wrong here
NAT is for outbound traffic, not for solving overlapping CIDRs in peering.
✗
Create the peering connection and add routes for the full CIDR ranges in both route tables.
Why it's wrong here
Overlapping CIDRs cause routing conflicts; peering may not work.
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: Verify that the CIDR blocks do not overlap. If they do, you must re-IP one VPC or use other solutions like Transit Gateway with network address translation. — Option B is correct because VPC peering does not support overlapping CIDRs; you cannot have routes to the same CIDR via different targets. If there is any overlap, you cannot create the peering connection or the routes. Option A is wrong because it suggests ignoring overlap. Option C is wrong because NAT does not solve routing ambiguity. Option D is wrong because Transit Gateway also has limitations with overlapping CIDRs.
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.
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