The answer is to deploy a Private NAT Gateway in the VPC and configure routes to send on-premises traffic through it. This solution resolves overlapping CIDR conflicts by translating the VPC’s 10.0.0.0/16 addresses into a unique, non-overlapping IP range before traffic traverses the Direct Connect link, allowing seamless communication with the on-premises network that uses the broader 10.0.0.0/8 range. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of how to handle overlapping RFC 1918 addresses in hybrid connectivity, a common trap where candidates mistakenly choose Transit Gateway or VPC Peering—both of which fail with overlapping CIDRs without additional NAT. A key memory tip is “NAT before the gate”: when IPs clash, a Private NAT Gateway must sit between your VPC and the Direct Connect Gateway to translate addresses, ensuring the on-premises router sees only the translated range.
ANS-C01 Network Design Practice Question
This ANS-C01 practice question tests your understanding of network design. 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 an IPv4 CIDR of 10.0.0.0/16. They need to connect their on-premises data center to AWS using AWS Direct Connect. The data center uses RFC 1918 addresses from the 10.0.0.0/8 range, overlapping with the VPC CIDR. The company cannot change the on-premises IP addresses. Which design allows connectivity without IP conflicts?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Deploy a Private NAT Gateway in the VPC and configure routes to send on-premises traffic through it
Option B is correct because Private NATGateway can translate overlapping VPC IPs to a different CIDR for communication with on-premises. Option A is wrong because VPC Peering does not support overlapping CIDRs. Option C is wrong because Transit Gateway also cannot handle overlapping CIDRs without NAT. Option D is wrong because Direct Connect Gateway alone does not resolve overlapping IPs.
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.
✗
Configure Direct Connect Gateway with network address translation
Why it's wrong here
Direct Connect Gateway does not provide NAT functionality.
✗
Use AWS Transit Gateway with route table separation
Why it's wrong here
Transit Gateway does not resolve IP overlap; separate route tables do not prevent conflicts.
✓
Deploy a Private NAT Gateway in the VPC and configure routes to send on-premises traffic through it
Why this is correct
Private NATGateway translates VPC IPs to a different range, allowing communication with overlapping on-premises addresses.
Set up VPC Peering between the VPC and on-premises network over Direct Connect
Why it's wrong here
VPC Peering requires non-overlapping CIDRs and cannot be used with on-premises networks.
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 Design — This question tests Network Design — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: Deploy a Private NAT Gateway in the VPC and configure routes to send on-premises traffic through it — Option B is correct because Private NAT Gateway can translate overlapping VPC IPs to a different CIDR for communication with on-premises. Option A is wrong because VPC Peering does not support overlapping CIDRs. Option C is wrong because Transit Gateway also cannot handle overlapping CIDRs without NAT. Option D is wrong because Direct Connect Gateway alone does not resolve overlapping IPs.
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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Question Discussion
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