- A
Create an ingress firewall rule on the backend VPC with source service account 'frontend-sa@project.iam.gserviceaccount.com', protocol tcp:8080, and target service account 'backend-sa@project.iam.gserviceaccount.com'.
Using source and target service accounts precisely restricts traffic to only the frontend service account communicating to the backend service account on tcp:8080.
- B
Create an egress firewall rule on the frontend VPC with source CIDR 10.0.1.0/24, protocol tcp:8080, and target CIDR 10.0.2.0/24.
Why wrong: Egress rules apply to outgoing traffic; this would allow all traffic from 10.0.1.0/24 to 10.0.2.0/24, but that is overly broad and does not use service accounts.
- C
Create an ingress firewall rule on the backend VPC with source CIDR 10.0.1.0/24, protocol tcp:8080, and target tag 'backend-tag'.
This rule restricts incoming traffic to the backend subnet from the frontend subnet CIDR on tcp:8080, effectively achieving the goal.
- D
Create an ingress firewall rule on the backend VPC with source tag 'frontend-tag', protocol tcp:8080, and target tag 'backend-tag'.
Why wrong: Source tags refer to the tags on the source instances, but firewall rules evaluate source tags only when the rule is applied to the instance's network. This rule would allow traffic from any instance with tag 'frontend-tag', not just from the frontend subnet.
- E
Create a VPC firewall rule with priority 1000 that denies all traffic from 10.0.1.0/24 to 10.0.2.0/24, and then a higher priority rule allowing tcp:8080.
Why wrong: This approach uses deny rules which are generally not recommended because they complicate firewall rule management and can lead to unexpected blocks.
Quick Answer
The correct answer is to create an ingress firewall rule on the backend VPC with source service account 'frontend-sa@...' and target service account 'backend-sa@...', allowing TCP port 8080. This configuration is correct because service account-based firewall rules operate at the instance identity level, not the IP level, meaning the rule dynamically permits traffic from any frontend instance using the specified service account, regardless of its source IP or subnet. On the Google Professional Cloud Network Engineer exam, this tests your understanding of VPC firewall rules with service accounts as an alternative to CIDR-based rules, a common trap being that many candidates mistakenly rely on subnet ranges or network tags when service accounts provide more granular, identity-aware control for inter-VPC traffic. A key memory tip is "service accounts are identities, not addresses"—think of the rule as checking who the instance is, not where it lives, which makes it ideal for dynamic environments where IPs may change.
PCNE Implementing network security Practice Question
This PCNE practice question tests your understanding of implementing network security. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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 is deploying a new application across three VPCs in the same project, using Shared VPC. The security team wants to restrict traffic such that only the frontend subnet (10.0.1.0/24) can send traffic to the backend subnet (10.0.2.0/24) on TCP port 8080. The backend instances have the service account 'backend-sa@project.iam.gserviceaccount.com'. Which TWO firewall rule configurations achieve this goal?
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
Create an ingress firewall rule on the backend VPC with source service account 'frontend-sa@project.iam.gserviceaccount.com', protocol tcp:8080, and target service account 'backend-sa@project.iam.gserviceaccount.com'.
Option A is correct because it uses service accounts as both source and target in an ingress rule on the backend VPC. This allows only instances with the frontend service account to send traffic to instances with the backend service account on TCP 8080, meeting the security requirement without relying on IP addresses or network tags.
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.
- ✓
Create an ingress firewall rule on the backend VPC with source service account 'frontend-sa@project.iam.gserviceaccount.com', protocol tcp:8080, and target service account 'backend-sa@project.iam.gserviceaccount.com'.
Why this is correct
Using source and target service accounts precisely restricts traffic to only the frontend service account communicating to the backend service account on tcp:8080.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Create an egress firewall rule on the frontend VPC with source CIDR 10.0.1.0/24, protocol tcp:8080, and target CIDR 10.0.2.0/24.
Why it's wrong here
Egress rules apply to outgoing traffic; this would allow all traffic from 10.0.1.0/24 to 10.0.2.0/24, but that is overly broad and does not use service accounts.
- ✓
Create an ingress firewall rule on the backend VPC with source CIDR 10.0.1.0/24, protocol tcp:8080, and target tag 'backend-tag'.
- ✗
Create an ingress firewall rule on the backend VPC with source tag 'frontend-tag', protocol tcp:8080, and target tag 'backend-tag'.
Why it's wrong here
Source tags refer to the tags on the source instances, but firewall rules evaluate source tags only when the rule is applied to the instance's network. This rule would allow traffic from any instance with tag 'frontend-tag', not just from the frontend subnet.
- ✗
Create a VPC firewall rule with priority 1000 that denies all traffic from 10.0.1.0/24 to 10.0.2.0/24, and then a higher priority rule allowing tcp:8080.
Why it's wrong here
This approach uses deny rules which are generally not recommended because they complicate firewall rule management and can lead to unexpected blocks.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume egress rules on the source VPC are sufficient to control inbound traffic to the backend, but Google Cloud requires ingress rules on the destination VPC to filter incoming packets, and service account-based rules are often overlooked in favor of IP-based rules.
Detailed technical explanation
How to think about this question
In Google Cloud VPC, ingress firewall rules are evaluated on the destination VPC, and service account-based rules use the IAM identity of the source or target instance, not the network layer. This allows fine-grained access control even if IP addresses change, which is critical in dynamic environments like autoscaling groups. The rule in Option A uses source service account 'frontend-sa' and target service account 'backend-sa', ensuring only instances with those specific identities can communicate, regardless of subnet placement.
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.
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FAQ
Questions learners often ask
What does this PCNE question test?
Implementing network security — This question tests Implementing network security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Create an ingress firewall rule on the backend VPC with source service account 'frontend-sa@project.iam.gserviceaccount.com', protocol tcp:8080, and target service account 'backend-sa@project.iam.gserviceaccount.com'. — Option A is correct because it uses service accounts as both source and target in an ingress rule on the backend VPC. This allows only instances with the frontend service account to send traffic to instances with the backend service account on TCP 8080, meeting the security requirement without relying on IP addresses or network tags.
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.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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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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