Resolving Overlapping CIDRs for VPC Peering and Transit Gateway
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.)
Quick Answer
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.”
⚠ Common exam trap
The ANS-C01 exam often tests the misconception that overlapping CIDRs can be resolved by simply updating route tables, but AWS strictly prohibits VPC peering with overlapping CIDRs, making a CIDR change mandatory before any routing can be configured.
Answer choices
Why each option matters
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.
VPC peering requires non-overlapping CIDR blocks. Since both VPCs use 10.0.0.0/16, they have identical address spaces, which makes routing ambiguous and prevents the peering connection from being established. AWS enforces this rule to ensure that route tables can uniquely direct traffic between the peered VPCs without conflicts.
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.
Why this is correct
Overlapping CIDRs prevent direct peering.
- ✗
Create VPC endpoints in each VPC for the other VPC's services.
Why it's wrong here
Not needed for VPC peering.
- ✗
Set up a VPN connection between the VPCs.
Why it's wrong here
Setting up a VPN connection is incorrect because standard VPNs, like VPC peering, cannot establish connectivity between VPCs that have identical CIDR blocks (10.0.0.0/16), as this creates ambiguous routing. VPN connections are, however, a valid solution for securely connecting networks with non-overlapping CIDRs, such as an on-premises data centre to a VPC, or between VPCs in different regions, or for remote user access via AWS Client VPN.
- ✓
Create a VPC peering connection request from one account and accept it in the other.
Why this is correct
Standard peering process.
- ✓
Update route tables in both VPCs to add routes for the peered VPC CIDR.
Why this is correct
Required for traffic to flow.
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Same concept, more angles
4 more ways this is tested on ANS-C01
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: 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: AWS VPC peering requires that the CIDR blocks of the two VPCs do not overlap. When both VPCs use the same IP range (10.0.0.0/16 and 10.0.0.0/24), the peering connection request will fail because overlapping CIDRs create routing ambiguity and are not supported by the VPC peering service. This is a hard limit enforced at the time of peering creation, not during route table configuration.
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: VPC peering requires non-overlapping CIDR blocks because the route tables in each VPC must have distinct destination prefixes to route traffic correctly. When both VPCs use 10.0.0.0/16, the routes are identical, causing ambiguity and preventing the peering connection from being established. The only resolution is to change the CIDR of one VPC or use a NAT-based solution to translate addresses.
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.
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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