- A
The firewall rules block all outbound traffic from the subnet
Why wrong: Firewall rules are stateful and allow outbound traffic by default, but Cloud NAT still requires proper tagging.
- B
The instances are using a custom network that does not support Cloud NAT
Why wrong: Cloud NAT works with custom VPC networks.
- C
The instances are not tagged with the correct network tag used in the Cloud NAT configuration
Cloud NAT uses tags or service accounts to determine which instances can use it for outbound connectivity.
- D
The Cloud NAT gateway uses dynamic port allocation, which is disabled
Why wrong: Dynamic port allocation is the default and does not affect connectivity.
Question 835 of 982
PCNE Implementing VPC Instances Practice Question
This PCNE practice question tests your understanding of implementing vpc instances. 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.
An engineer needs to provide outbound internet access to a set of Compute Engine instances that do not have external IP addresses. The instances are in a VPC subnet with a Cloud NAT configured. However, the instances still cannot reach the internet. The engineer verified that Cloud NAT is configured on the same region and VPC as the instances. 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.
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 instances are not tagged with the correct network tag used in the Cloud NAT configuration
Cloud NAT requires the instances to have the 'allow NAT' network tag or service account configured in the NAT gateway rules. If the instances are not tagged or the service account is not specified, traffic is not forwarded.
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 firewall rules block all outbound traffic from the subnet
- ✗
The instances are using a custom network that does not support Cloud NAT
Why it's wrong here
Cloud NAT works with custom VPC networks.
- ✓
The instances are not tagged with the correct network tag used in the Cloud NAT configuration
- ✗
The Cloud NAT gateway uses dynamic port allocation, which is disabled
Why it's wrong here
Dynamic port allocation is the default and does not affect connectivity.
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.
Visual reference
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 PCNE subnetting questions on CIDR, address ranges, and subnet selection.
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Implementing VPC Instances — study guide chapter
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FAQ
Questions learners often ask
What does this PCNE question test?
Implementing VPC Instances — This question tests Implementing VPC Instances — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The instances are not tagged with the correct network tag used in the Cloud NAT configuration — Cloud NAT requires the instances to have the 'allow NAT' network tag or service account configured in the NAT gateway rules. If the instances are not tagged or the service account is not specified, traffic is not forwarded.
What should I do if I get this PCNE 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 PCNE subnetting questions on CIDR, address ranges, and subnet selection.
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?
CIDR notation defines the prefix length.
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Last reviewed: Jul 4, 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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