- A
Cloud NAT is configured with endpoint-independent mapping; change to endpoint-dependent mapping to ensure consistent source IP.
Why wrong: Endpoint-independent mapping is the default and does not affect source IP consistency.
- B
Cloud NAT is configured with dynamic port allocation; use static port allocation instead.
Why wrong: Dynamic port allocation affects port mapping, not source IP consistency.
- C
Cloud NAT is using a manual NAT IP address that is not assigned to the instances; assign the NAT IP to the instances as an alias IP range.
Why wrong: NAT IPs are not assigned to instances; they are used by Cloud NAT for outbound traffic.
- D
Cloud NAT is configured with a default rule that does not include the subnet; add a custom NAT rule that specifically includes subnet 10.1.0.0/20.
If the subnet is not in a NAT rule, instances may not use NAT or use different NAT IPs, causing inconsistent source IPs. Adding the subnet ensures consistent NAT IP usage.
Quick Answer
The answer is to add a custom NAT rule that specifically includes subnet 10.1.0.0/20. This is correct because Cloud NAT applies translation based on rules, and if the default rule does not include a given subnet, instances in that subnet will either use their ephemeral public IPs or fail to reach external endpoints, causing the inconsistent source IP addresses seen by the whitelisted API. On the Google Professional Cloud Network Engineer exam, this scenario tests your understanding that Cloud NAT rules are evaluated in order, and a default rule that omits a subnet is a common misconfiguration trap—candidates often assume all subnets are automatically covered. The key insight is that Cloud NAT requires explicit subnet inclusion for traffic to be translated through the NAT gateway’s consistent IP. Memory tip: “If it’s not in the rule, it’s not in the NAT”—always verify your subnet list when troubleshooting source IP inconsistency.
PCNE Configuring network services Practice Question
This PCNE practice question tests your understanding of configuring network services. 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 deployed a web application on Compute Engine instances in a VPC with subnet 10.1.0.0/20. The instances need to access an external API that whitelists IP addresses. The company uses Cloud NAT to provide outbound connectivity. The API integration tests are failing, and the operations team suspects that the source IP addresses seen by the API are not consistent. What is the most likely cause and solution?
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.
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
Cloud NAT is configured with a default rule that does not include the subnet; add a custom NAT rule that specifically includes subnet 10.1.0.0/20.
Option D is correct because if Cloud NAT's default rule does not include the subnet 10.1.0.0/20, instances in that subnet will not have their outbound traffic translated through the NAT gateway, causing them to use their ephemeral public IPs (if any) or fail to reach the external API. Adding a custom NAT rule that explicitly includes the subnet ensures all outbound traffic from those instances uses the consistent NAT IP address that the API whitelist expects.
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.
- ✗
Cloud NAT is configured with endpoint-independent mapping; change to endpoint-dependent mapping to ensure consistent source IP.
Why it's wrong here
Endpoint-independent mapping is the default and does not affect source IP consistency.
- ✗
Cloud NAT is configured with dynamic port allocation; use static port allocation instead.
Why it's wrong here
Dynamic port allocation affects port mapping, not source IP consistency.
- ✗
Cloud NAT is using a manual NAT IP address that is not assigned to the instances; assign the NAT IP to the instances as an alias IP range.
- ✓
Cloud NAT is configured with a default rule that does not include the subnet; add a custom NAT rule that specifically includes subnet 10.1.0.0/20.
Why this is correct
If the subnet is not in a NAT rule, instances may not use NAT or use different NAT IPs, causing inconsistent source IPs. Adding the subnet ensures consistent NAT IP usage.
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.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume Cloud NAT automatically applies to all subnets in the VPC, but in reality, the default rule must explicitly include the subnet, and if it is removed or not configured, traffic from that subnet will not be NATed.
Detailed technical explanation
How to think about this question
Cloud NAT uses source network address translation (SNAT) to map private IPs of VMs to a single public IP (or a pool) for outbound traffic. The NAT rule configuration determines which subnets are eligible for translation; if a subnet is not included, traffic from that subnet either uses an ephemeral external IP (if the VM has one) or is dropped. The default rule in Cloud NAT applies to all primary and secondary IP ranges of subnets in the region, but if it is deleted or modified, explicit rules must be added. This is a common misconfiguration when deploying Cloud NAT after VPC creation, as the default rule may not automatically cover newly added subnets.
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.
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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Configuring network services — study guide chapter
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FAQ
Questions learners often ask
What does this PCNE question test?
Configuring network services — This question tests Configuring network services — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Cloud NAT is configured with a default rule that does not include the subnet; add a custom NAT rule that specifically includes subnet 10.1.0.0/20. — Option D is correct because if Cloud NAT's default rule does not include the subnet 10.1.0.0/20, instances in that subnet will not have their outbound traffic translated through the NAT gateway, causing them to use their ephemeral public IPs (if any) or fail to reach the external API. Adding a custom NAT rule that explicitly includes the subnet ensures all outbound traffic from those instances uses the consistent NAT IP address that the API whitelist expects.
What should I do if I get this PCNE 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.
About these practice questions
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Last reviewed: Jun 24, 2026
This PCNE practice question is part of Courseiva's free Google Cloud 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 PCNE exam.
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