- A
Create a VPC route table that routes all traffic (0.0.0.0/0) to the Transit Gateway.
Why wrong: This only affects internet-bound traffic, not intra-VPC traffic.
- B
Create a VPC route table that routes the VPC CIDR to the Transit Gateway and associate it with each subnet.
This forces all traffic destined for other subnets to go to the Transit Gateway for inspection.
- C
Use the main route table and add a route for the VPC CIDR to the Transit Gateway.
Why wrong: The main route table's local route for the VPC CIDR cannot be overridden; traffic between subnets will remain local.
- D
Associate each subnet with a custom route table that has a route for the VPC CIDR to the Transit Gateway.
Why wrong: The local route for the VPC CIDR cannot be overridden; the route to the Transit Gateway would be ignored.
Quick Answer
The answer is to create a VPC route table that routes the VPC CIDR to the Transit Gateway and associate it with each subnet. This works because by default, AWS uses a local route for intra-VPC traffic, meaning traffic between subnets never leaves the VPC and thus bypasses any inspection appliance. By overriding that local route with a more specific or equal route pointing to the Transit Gateway, you force all inter-subnet traffic to exit the VPC, traverse the Transit Gateway, and reach the centralized inspection VPC before being forwarded to its destination. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this concept tests your understanding of route priority and the limitations of VPC peering versus Transit Gateway for traffic inspection. A common trap is assuming that simply attaching subnets to a Transit Gateway automatically routes inter-subnet traffic through it—it does not, because the local route remains active. Memory tip: think “local is the default jail; to escape inspection, you must route the entire VPC CIDR to the TGW.”
ANS-C01 Network Design Practice Question
This ANS-C01 practice question tests your understanding of network design. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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 multiple subnets and an AWS Transit Gateway. They have a requirement to inspect traffic between subnets using a third-party firewall appliance that is deployed in a centralized inspection VPC. The firewall appliance must process all traffic between the VPC subnets, including traffic between subnets in the same Availability Zone. Which routing configuration achieves this?
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
Create a VPC route table that routes the VPC CIDR to the Transit Gateway and associate it with each subnet.
Option D is correct. To force all inter-subnet traffic through the Transit Gateway, you must create a VPC route table that routes traffic destined for other subnets (the VPC's CIDR) to the Transit Gateway. This ensures traffic leaves the VPC and goes through the Transit Gateway to the inspection VPC. Option A is incorrect because the default route for intra-VPC traffic is local, which would bypass the Transit Gateway. Option B is incorrect because the main route table still has the local route for the VPC CIDR, so traffic between subnets stays within the VPC. Option C is incorrect because subnet route tables are attached to subnets, but if the route tables have the local route for the VPC CIDR, traffic will not go to the Transit Gateway.
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.
- ✗
Create a VPC route table that routes all traffic (0.0.0.0/0) to the Transit Gateway.
Why it's wrong here
This only affects internet-bound traffic, not intra-VPC traffic.
- ✓
Create a VPC route table that routes the VPC CIDR to the Transit Gateway and associate it with each subnet.
- ✗
Use the main route table and add a route for the VPC CIDR to the Transit Gateway.
Why it's wrong here
The main route table's local route for the VPC CIDR cannot be overridden; traffic between subnets will remain local.
- ✗
Associate each subnet with a custom route table that has a route for the VPC CIDR to the Transit Gateway.
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 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.
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FAQ
Questions learners often ask
What does this ANS-C01 question test?
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: Create a VPC route table that routes the VPC CIDR to the Transit Gateway and associate it with each subnet. — Option D is correct. To force all inter-subnet traffic through the Transit Gateway, you must create a VPC route table that routes traffic destined for other subnets (the VPC's CIDR) to the Transit Gateway. This ensures traffic leaves the VPC and goes through the Transit Gateway to the inspection VPC. Option A is incorrect because the default route for intra-VPC traffic is local, which would bypass the Transit Gateway. Option B is incorrect because the main route table still has the local route for the VPC CIDR, so traffic between subnets stays within the VPC. Option C is incorrect because subnet route tables are attached to subnets, but if the route tables have the local route for the VPC CIDR, traffic will not go to the Transit Gateway.
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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Last reviewed: Jun 20, 2026
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