The answer is a target service account mismatch, because the firewall rule applies only to VMs associated with that specific service account, and the VM at 10.0.0.5 likely has a different or no service account assigned. In GCP, when a firewall rule specifies a target service account, it overrides any target tags or network-level targeting, meaning the rule’s allow action is scoped exclusively to instances using that exact account. If the destination VM lacks that account, the implicit deny-all egress and ingress rules block the traffic, even though both VMs are in the same default VPC. On the Google Professional Cloud Network Engineer exam, this scenario tests your understanding of how service account-based firewall rules differ from tag-based rules—a common trap is assuming all VPC traffic is allowed by default. Remember the mnemonic: “Account mismatch blocks the switch”—if the target service account doesn’t match the VM’s identity, the firewall rule simply won’t apply.
PCNE Practice Question: Designing, planning, and prototyping a GCP network
This PCNE practice question tests your understanding of designing, planning, and prototyping a gcp network. 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.
Refer to the exhibit. A VM in the default VPC with IP 10.0.0.5 is unable to receive traffic from another VM in the same VPC with IP 10.0.1.5. The firewall rule shown is in place. What is the most likely reason?
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 the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The target service account does not match the VM's service account
The firewall rule shown uses a target service account, which means it applies only to VM instances that are associated with that specific service account. If the VM at 10.0.0.5 has a different service account (or no service account) than the one specified in the rule, the rule will not apply to it, and traffic will be blocked by the implicit deny-all egress/ingress firewall rules. This is the most likely reason the VM cannot receive traffic from 10.0.1.5.
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 source range does not include 10.0.1.5
Why it's wrong here
10.0.1.5 is within 10.0.0.0/8.
✗
The rule only allows TCP but the traffic is UDP
Why it's wrong here
The rule allows all TCP ports; if traffic is UDP, it would be blocked, but the stem implies TCP (any port) is being used.
✓
The target service account does not match the VM's service account
Why this is correct
The rule only applies to VMs with the specified service account.
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 priority is too low
Why it's wrong here
Priority 1000 is default; no higher priority rule is shown.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the distinction between target tags and target service accounts, and the trap here is that candidates assume a firewall rule with a broad source range (0.0.0.0/0) will apply to all VMs, overlooking that the target service account field restricts which VMs the rule actually applies to.
Trap categories for this question
Command / output trap
Priority 1000 is default; no higher priority rule is shown.
Detailed technical explanation
How to think about this question
In Google Cloud VPC, firewall rules can be applied to VMs using either target tags or target service accounts. When a target service account is specified, the rule only applies to VM instances that have that exact service account attached. If no service account is specified in the rule, it applies to all instances in the network. The implicit deny ingress rule blocks all incoming traffic unless a higher-priority allow rule explicitly matches the traffic. This design allows fine-grained access control based on identity rather than just network attributes.
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.
Designing, planning, and prototyping a GCP network — This question tests Designing, planning, and prototyping a GCP network — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The target service account does not match the VM's service account — The firewall rule shown uses a target service account, which means it applies only to VM instances that are associated with that specific service account. If the VM at 10.0.0.5 has a different service account (or no service account) than the one specified in the rule, the rule will not apply to it, and traffic will be blocked by the implicit deny-all egress/ingress firewall rules. This is the most likely reason the VM cannot receive traffic from 10.0.1.5.
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.
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Question Discussion
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