The answer is that the VPC route table must have a route to the on-premises CIDR pointing to the transit gateway attachment, the transit gateway route table must include the static route for 172.16.0.0/12 pointing to the VPN attachment, and the VPC must be attached to the transit gateway. These three prerequisites ensure that traffic originating in the VPC is directed toward the transit gateway, which then uses its own route table to forward packets to the correct VPN attachment, and finally the attachment itself must exist to carry traffic into the tunnel. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this question tests your understanding of the full data path for Transit Gateway routing prerequisites, often trapping candidates who assume BGP is mandatory or that the VPN tunnel configuration alone suffices. A common memory tip is to think of the three necessary hops: VPC route table, transit gateway route table, and attachment—each must be explicitly configured for traffic to flow.
ANS-C01 Network Implementation Practice Question
This ANS-C01 practice question tests your understanding of network implementation. 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 needs to connect to an on-premises network with CIDR 172.16.0.0/12. They are using an AWS Transit Gateway with a VPN attachment to the on-premises network. The transit gateway route table has a static route for 172.16.0.0/12 pointing to the VPN attachment. Which THREE of the following are necessary for traffic to flow from the VPC to on-premises?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The VPC route tables include a route for 172.16.0.0/12 pointing to the transit gateway
Option A is necessary because the VPC route table must have a route to on-premises via the transit gateway. Option B is necessary because the transit gateway route table must have the route. Option C is necessary because the VPC must be attached to the transit gateway. Option D is not necessary because the VPN tunnel may be inside the transit gateway attachment. Option E is not necessary because BGP may not be required if static routes are used.
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.
✓
The VPC route tables include a route for 172.16.0.0/12 pointing to the transit gateway
Why this is correct
VPC instances need a route to send traffic to the transit gateway.
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: The VPC route tables include a route for 172.16.0.0/12 pointing to the transit gateway — Option A is necessary because the VPC route table must have a route to on-premises via the transit gateway. Option B is necessary because the transit gateway route table must have the route. Option C is necessary because the VPC must be attached to the transit gateway. Option D is not necessary because the VPN tunnel may be inside the transit gateway attachment. Option E is not necessary because BGP may not be required if static routes are used.
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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