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Network Design →hardMultiple ChoiceObjective-mapped

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 multi-VPC architecture connected via AWS Transit Gateway. They have VPCs in different AWS accounts. The network team wants to centralize internet traffic through a single egress VPC that has a NAT Gateway and an Internet Gateway. All other VPCs should route outbound internet traffic through the Transit Gateway to the egress VPC. They have configured route tables accordingly, but instances in non-egress VPCs cannot reach the internet. What is the most likely missing configuration?

Clue words in this question

Noticing these words before you look at the options changes how you read each choice.

Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.

Question 1hardmultiple choice

Read the full NAT/PAT explanation →

A
The Transit Gateway route table does not have a route to the non-egress VPC attachments for the return traffic.
For return traffic from the egress VPC to reach the non-egress VPCs, the TGW route table must have routes to those attachments.
B
The security group for the NAT Gateway in the egress VPC is blocking inbound traffic from other VPCs.
Why wrong: NAT Gateway does not have security groups.
C
The egress VPC's route table does not have a route to the Transit Gateway for the return traffic from the internet.
Why wrong: Return traffic from internet goes to NAT Gateway, which forwards to private instances; no need for route to TGW for return traffic from internet.
D
The non-egress VPCs are using VPC endpoints instead of the Transit Gateway for internet access.
Why wrong: VPC endpoints are for specific services, not general internet.

Full breakdown with real-world context →

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

✓

The Transit Gateway route table does not have a route to the non-egress VPC attachments for the return traffic.

Option D is correct. For Transit Gateway to forward traffic from one VPC to another, attachment routes must be propagated and route tables must allow transitive routing. Also, the egress VPC must have a route to the Transit Gateway for return traffic. Option A is incorrect because security groups are stateful. Option B is incorrect because NAT Gateway does not need routes back to Transit Gateway; it sends traffic to IGW. Option C is incorrect because VPC endpoints are not for general internet.

Key principle: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.

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 Transit Gateway route table does not have a route to the non-egress VPC attachments for the return traffic.
Why this is correct
For return traffic from the egress VPC to reach the non-egress VPCs, the TGW route table must have routes to those attachments.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Static NAT maps one inside address to one outside address.
✗
The security group for the NAT Gateway in the egress VPC is blocking inbound traffic from other VPCs.
Why it's wrong here
NAT Gateway does not have security groups.
✗
The egress VPC's route table does not have a route to the Transit Gateway for the return traffic from the internet.
Why it's wrong here
Return traffic from internet goes to NAT Gateway, which forwards to private instances; no need for route to TGW for return traffic from internet.
✗
The non-egress VPCs are using VPC endpoints instead of the Transit Gateway for internet access.
Why it's wrong here
VPC endpoints are for specific services, not general internet.

Common exam traps

Common exam trap: NAT rules depend on direction and matching traffic

NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.

Detailed technical explanation

How to think about this question

NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.

KKey Concepts to Remember

Static NAT maps one inside address to one outside address.
PAT allows many inside hosts to share one public address using ports.
Inside local and inside global describe the private and translated addresses.
NAT ACLs identify traffic for translation, not always security filtering.

TExam Day Tips

Identify inside and outside interfaces first.
Check whether the scenario needs static NAT, dynamic NAT or PAT.
Do not confuse NAT matching ACLs with normal packet-filtering intent.

Key takeaway

NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.

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 the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related ANS-C01 NAT questions on configuration and troubleshooting.

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FAQ

Questions learners often ask

What does this ANS-C01 question test?

Network Design — This question tests Network Design — Static NAT maps one inside address to one outside address..

What is the correct answer to this question?

The correct answer is: The Transit Gateway route table does not have a route to the non-egress VPC attachments for the return traffic. — Option D is correct. For Transit Gateway to forward traffic from one VPC to another, attachment routes must be propagated and route tables must allow transitive routing. Also, the egress VPC must have a route to the Transit Gateway for return traffic. Option A is incorrect because security groups are stateful. Option B is incorrect because NAT Gateway does not need routes back to Transit Gateway; it sends traffic to IGW. Option C is incorrect because VPC endpoints are not for general internet.

What should I do if I get this ANS-C01 question wrong?

Are there clue words in this question I should notice?

Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.

What is the key concept behind this question?

Static NAT maps one inside address to one outside address.

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