Quick Answer
The best way to avoid IP address overlap when connecting a VPC to an on-premises network via VPN is to use a different RFC 1918 CIDR block for the VPC, such as 172.16.0.0/12 or 192.168.0.0/16, rather than reusing the 10.0.0.0/8 space. This is critical because overlapping IP ranges prevent proper route propagation in the VPN tunnel, leading to asymmetric routing or complete connectivity failures, as both sides would claim the same addresses. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of VPC CIDR design principles and the pitfalls of overlapping private IPs, often appearing as a trap where candidates consider NAT as a simpler fix. While NAT can technically resolve overlap, it adds operational complexity and is not the best design choice when a non-overlapping private range is available. Memory tip: "Don't double-dip the 10—pick a different private strip."
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 is designing a network for a new VPC. They want to ensure that the VPC can connect to an on-premises data center via a site-to-site VPN. The on-premises network uses a CIDR block of 10.0.0.0/8. The VPC will use a CIDR block of 10.0.0.0/16. The network engineer is concerned about overlapping IP addresses. What is the best way to avoid IP address overlap?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "best"
Why it matters: Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
Use the same CIDR block and rely on NAT
Why wrong: Overlap will cause routing issues.
Use a different RFC 1918 CIDR block for the VPC
Avoids overlap.
Use a smaller subnet within the same 10.0.0.0/8 range
Why wrong: Still overlaps.
Use a public IP range for the VPC
Why wrong: Public IPs are not recommended for private VPCs.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Use a different RFC 1918 CIDR block for the VPC
Option B is correct because using a different RFC 1918 CIDR block (e.g., 172.16.0.0/12 or 192.168.0.0/16) for the VPC completely avoids IP address overlap with the on-premises 10.0.0.0/8 network. Overlapping CIDR blocks (both using 10.0.0.0/8) would prevent successful route propagation and cause asymmetric routing or connectivity failures in the site-to-site VPN. NAT can mitigate overlap but adds complexity and is not the 'best' design choice when a non-overlapping private range is available.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
Use the same CIDR block and rely on NAT
Why it's wrong here
Overlap will cause routing issues.
Use a different RFC 1918 CIDR block for the VPC
Why this is correct
Avoids overlap.
Clue confirmation
The clue word "best" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
Use a smaller subnet within the same 10.0.0.0/8 range
Why it's wrong here
Still overlaps.
Use a public IP range for the VPC
Why it's wrong here
Public IPs are not recommended for private VPCs.
Common exam traps
AWS often tests the misconception that NAT can always solve IP overlap issues, but the best practice is to design non-overlapping private IP spaces from the start to avoid complexity and routing failures.
Detailed technical explanation
Under the hood, AWS site-to-site VPN uses BGP dynamic routing or static routes; overlapping CIDRs cause the VPN connection to reject or ignore routes that conflict with the VPC's main route table, leading to unreachable destinations. In a real-world scenario, if both sides use 10.0.0.0/8, traffic destined for the VPC's 10.0.1.0/24 might be sent to the on-premises network instead, breaking connectivity. AWS recommends using unique RFC 1918 ranges per VPC and on-premises to ensure proper route propagation and avoid NAT overhead.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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.
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FAQ
Network Design — This question tests Network Design — Read the scenario before looking for a memorised answer..
The correct answer is: Use a different RFC 1918 CIDR block for the VPC — Option B is correct because using a different RFC 1918 CIDR block (e.g., 172.16.0.0/12 or 192.168.0.0/16) for the VPC completely avoids IP address overlap with the on-premises 10.0.0.0/8 network. Overlapping CIDR blocks (both using 10.0.0.0/8) would prevent successful route propagation and cause asymmetric routing or connectivity failures in the site-to-site VPN. NAT can mitigate overlap but adds complexity and is not the 'best' design choice when a non-overlapping private range is available.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Yes — watch for: "best". Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 30, 2026
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.
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