The correct answer involves three steps: updating route tables in both VPCs to add routes for the peered VPC CIDR, initiating a VPC peering request from one account and accepting it from the other, and resolving the overlapping CIDRs—typically by changing one VPC’s CIDR block. This is necessary because when two VPCs share the same 10.0.0.0/16 range, the AWS VPC peering connection cannot distinguish traffic destined for the local VPC from traffic meant for the peered VPC, causing routing ambiguity. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of how to connect VPCs with overlapping CIDRs, a common challenge in multi-account architectures. A frequent trap is assuming a VPN or VPC endpoint can bypass the overlap, but those services do not resolve the fundamental routing conflict. Memory tip: “Peering needs unique CIDRs—if they clash, change one or use a Transit Gateway with network address translation.”
ANS-C01 Network Design Practice Question
This ANS-C01 practice question tests your understanding of network design. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. 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 wants to connect to another VPC with CIDR 10.0.0.0/16 in a different account. The VPCs are in the same region. Which THREE steps are necessary to establish connectivity? (Choose THREE.)
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Change the CIDR block of one VPC to a non-overlapping range.
Option A is correct: Overlapping CIDRs must be resolved, typically by changing one VPC's CIDR. Option B is correct: VPC peering requires a request from one account and acceptance from the other. Option D is correct: Route tables must be updated to route traffic to the peering connection. Option C is wrong because a VPN is unnecessary. Option E is wrong because VPC endpoints are for accessing services, not for VPC peering.
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.
✓
Change the CIDR block of one VPC to a non-overlapping range.
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.
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 Design — This question tests Network Design — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: Change the CIDR block of one VPC to a non-overlapping range. — Option A is correct: Overlapping CIDRs must be resolved, typically by changing one VPC's CIDR. Option B is correct: VPC peering requires a request from one account and acceptance from the other. Option D is correct: Route tables must be updated to route traffic to the peering connection. Option C is wrong because a VPN is unnecessary. Option E is wrong because VPC endpoints are for accessing services, not for VPC peering.
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.
About these practice questions
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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 has a VPC with a CIDR of 10.0.0.0/16 and needs to connect to another VPC with CIDR 10.0.0.0/16 via VPC Peering. They encounter an error because of overlapping CIDRs. Which TWO actions can resolve this issue?
medium
A.Use a subnet-level peering connection
✓ B.Create a new VPC with a non-overlapping CIDR and peer with that
C.Modify the CIDR of one VPC to a non-overlapping range
✓ D.Use AWS Transit Gateway with separate route tables
E.Use a NAT Gateway to translate IPs
Why B: Option B is correct because creating a new VPC with a non-overlapping CIDR and establishing a VPC peering connection resolves the conflict. VPC peering requires that the CIDR blocks of the two VPCs do not overlap, as overlapping ranges prevent proper route table entries and cause routing ambiguity. By using a non-overlapping CIDR, you enable direct connectivity between the VPCs without IP address conflicts.
Variation 2. A company has a VPC with a CIDR of 10.0.0.0/16. They need to peer with another VPC that has a CIDR of 10.0.0.0/24. What will happen?
hard
A.The peering connection will be established, but only the first VPC's CIDR will be used.
B.The peering connection will be established, but routes with overlapping CIDRs will not be added automatically.
✓ C.The peering connection will fail because the CIDRs overlap.
D.The peering connection will be established, and the overlapping CIDRs will be ignored.
Why C: Option D is correct because overlapping CIDRs cannot be peered. Option A is wrong because VPC Peering does not support overlapping CIDRs. Option B is wrong because AWS will reject the peering request. Option C is wrong because overlapping CIDRs are not allowed.
Variation 3. A company has a VPC with a CIDR of 10.0.0.0/16 and needs to peer with another VPC that has CIDR 10.0.0.0/16. What is the issue and how can it be resolved?
easy
A.The VPCs can be peered if you enable DNS resolution.
✓ B.The VPCs have overlapping CIDRs, so they cannot be directly peered. You must re-IP one VPC or use a NAT solution.
C.Use a Transit Gateway to connect the VPCs, which supports overlapping CIDRs.
D.Use a Direct Connect Gateway to connect the VPCs, which ignores CIDR overlap.
Why B: Option C is correct because overlapping CIDRs prevent VPC Peering. Option A is wrong because VPC Peering does not require non-overlapping CIDRs for established connections but for new ones it's a limitation. Option B is wrong because Transit Gateway also does not support overlapping CIDRs without NAT. Option D is wrong because Direct Connect also requires non-overlapping CIDRs.
Variation 4. A company has a VPC with a CIDR of 10.0.0.0/16 and needs to peer with another VPC with CIDR 10.0.0.0/16. They plan to use a transit gateway to connect the VPCs. What is the correct approach to handle the overlapping CIDR ranges?
hard
✓ A.Create a new VPC with a non-overlapping CIDR and migrate resources from one of the existing VPCs.
B.Create a VPC peering connection and use a smaller subnet CIDR for traffic filtering.
C.Use a transit gateway with network address translation (NAT) to translate one VPC CIDR to a non-overlapping range.
D.Configure the transit gateway with equal-cost multipath (ECMP) to load balance traffic between the overlapping CIDRs.
Why A: Transit gateways do not perform NAT or resolve overlapping CIDR conflicts between attached VPCs. When two VPCs have identical CIDR blocks (10.0.0.0/16), the transit gateway cannot route traffic correctly because it cannot distinguish between the two networks. The only viable solution is to create a new VPC with a non-overlapping CIDR and migrate resources from one of the existing VPCs, eliminating the conflict at the network layer.
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