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Network Implementation →mediumMultiple ChoiceObjective-mapped

ANS-C01 Network Implementation Practice Question

This ANS-C01 practice question tests your understanding of network implementation. 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 private subnets that use a NAT gateway for outbound internet access. The NAT gateway is in a public subnet with an Elastic IP. Users report that some applications are failing to connect to external services. Network engineers confirm that the NAT gateway is in the 'available' state and the route tables have a default route (0.0.0.0/0) pointing to the NAT gateway. What is the most likely cause?

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.

A
The security group for the private instances is blocking outbound traffic.
Why wrong: Outbound traffic is allowed by default; the issue is return traffic.
B
The network ACL for the public subnet is blocking inbound traffic on ephemeral ports.
NACL must allow inbound traffic from the internet on ephemeral ports (1024-65535) for return traffic.
C
The NAT gateway has reached its connection limit.
Why wrong: NAT gateway supports up to 55,000 simultaneous connections, unlikely cause.
D
The route table for the private subnets is missing a route to the internet gateway.
Why wrong: Private subnets route to NAT gateway, not internet gateway.

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 network ACL for the public subnet is blocking inbound traffic on ephemeral ports.

The network ACL for the public subnet is blocking inbound traffic on ephemeral ports. When a private instance initiates outbound traffic through the NAT gateway, the NAT gateway sends the traffic to the internet and expects return traffic on high-numbered ephemeral ports (1024–65535). If the public subnet's network ACL blocks inbound traffic on these ports, the return traffic cannot reach the NAT gateway, causing asymmetric routing and connection failures. The NAT gateway itself is available and the route tables are correctly configured, so the issue lies in the stateful nature of security groups versus the stateless nature of network ACLs.

Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.

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 security group for the private instances is blocking outbound traffic.
Why it's wrong here
Outbound traffic is allowed by default; the issue is return traffic.
✓
The network ACL for the public subnet is blocking inbound traffic on ephemeral ports.
Why this is correct
NACL must allow inbound traffic from the internet on ephemeral ports (1024-65535) for return traffic.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The NAT gateway has reached its connection limit.
Why it's wrong here
NAT gateway supports up to 55,000 simultaneous connections, unlikely cause.
✗
The route table for the private subnets is missing a route to the internet gateway.
Why it's wrong here
Private subnets route to NAT gateway, not internet gateway.

Common exam traps

Common exam trap: answer the scenario, not the keyword

The ANS-C01 exam often tests the distinction between stateful security groups and stateless network ACLs, and the trap here is that candidates assume the NAT gateway's availability and correct route tables guarantee connectivity, overlooking the need for explicit inbound ephemeral port rules in the public subnet's network ACL.

Detailed technical explanation

How to think about this question

Network ACLs are stateless, meaning you must explicitly allow both outbound and inbound traffic for each direction. For a NAT gateway in a public subnet, the outbound ACL must allow traffic to the internet (e.g., 0.0.0.0/0 on ports 80, 443), and the inbound ACL must allow return traffic from the internet on ephemeral ports (1024–65535) with source 0.0.0.0/0. A common misconfiguration is to allow only outbound rules in the public subnet's ACL, forgetting the inbound ephemeral port rule, which causes intermittent failures for protocols like HTTP/HTTPS. This is a classic AWS networking pitfall because security groups (stateful) mask the need for explicit inbound rules in ACLs.

KKey Concepts to Remember

Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.

TExam Day Tips

Watch for words such as best, first, most likely and least administrative effort.
Review why wrong options are wrong, not only why the correct option is correct.

Key takeaway

Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.

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.

Visual reference

Quick reference

Asymmetric Encryption Algorithm Comparison

Algorithm	Key Exchange	Signatures	Equivalent Security Key	Notes
RSA-3072	Yes	Yes	128-bit	Widely deployed; slow for bulk data
ECDSA P-256	No	Yes	128-bit	Fast signatures; standard TLS certs
ECDH / ECDHE	Yes	No	128-bit	Perfect forward secrecy in TLS 1.3
DH / DHE	Yes	No	128-bit (3072-bit key)	Replaced by ECDHE in modern TLS
Ed25519	No	Yes	~128-bit	SSH keys, modern PKI

What to study next

Got this wrong? Here's your next step.

Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.

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FAQ

Questions learners often ask

What does this ANS-C01 question test?

Network Implementation — This question tests Network Implementation — Read the scenario before looking for a memorised answer..

What is the correct answer to this question?

The correct answer is: The network ACL for the public subnet is blocking inbound traffic on ephemeral ports. — The network ACL for the public subnet is blocking inbound traffic on ephemeral ports. When a private instance initiates outbound traffic through the NAT gateway, the NAT gateway sends the traffic to the internet and expects return traffic on high-numbered ephemeral ports (1024–65535). If the public subnet's network ACL blocks inbound traffic on these ports, the return traffic cannot reach the NAT gateway, causing asymmetric routing and connection failures. The NAT gateway itself is available and the route tables are correctly configured, so the issue lies in the stateful nature of security groups versus the stateless nature of network ACLs.

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

Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.

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?

Read the scenario before looking for a memorised answer.

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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.