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. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. 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 designing a VPC for a multi-tier application. The web tier must be accessible from the internet, the app tier only from the web tier, and the db tier only from the app tier. Which combination of firewall rules is appropriate?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Ingress allow from 0.0.0.0/0 to web, ingress allow from web subnets to app, ingress allow from app subnets to db
Option B is correct because it follows the principle of least privilege for a multi-tier VPC. The web tier must be accessible from the internet (0.0.0.0/0) on ingress, the app tier must only accept ingress from the web tier subnets, and the db tier must only accept ingress from the app tier subnets. This ensures that each tier is isolated and only reachable from the immediate upstream tier, which is a fundamental security best practice for multi-tier architectures in Google Cloud.
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.
✗
Ingress allow from web to app, ingress allow from app to db, no rule for web
Why it's wrong here
Web tier is not accessible from internet.
✓
Ingress allow from 0.0.0.0/0 to web, ingress allow from web subnets to app, ingress allow from app subnets to db
Why this is correct
Correctly restricts access at each tier.
Related concept
Read the scenario before looking for a memorised answer.
✗
Ingress allow from 0.0.0.0/0 to web, ingress allow from all subnets to app, ingress allow from app to db
Why it's wrong here
Allows any subnet to reach app, not just web.
✗
Ingress allow from web to app, ingress allow from web to db, ingress allow from app to db
Why it's wrong here
Allows web to directly access db.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse 'ingress allow from web to app' with 'ingress allow from web subnets to app', forgetting that firewall rules must specify source CIDR ranges (e.g., subnet IPs) rather than just the tier name, and they may also incorrectly allow direct web-to-db access, thinking it simplifies connectivity without realizing it breaks the isolation requirement.
Detailed technical explanation
How to think about this question
In Google Cloud VPC, firewall rules are stateful, meaning that if you allow ingress traffic, the corresponding egress response traffic is automatically allowed. However, for multi-tier isolation, you must explicitly restrict ingress rules to specific source ranges (e.g., web subnet CIDR for app tier) to prevent lateral movement. A common subtlety is that VPC firewall rules are evaluated in order of priority (lower number = higher priority), and if no rule matches, traffic is denied by default (implicit deny). In a real-world scenario, you would also consider using VPC Service Controls or Private Google Access for additional security, but the core isolation relies on precise source-based ingress rules.
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 media company stores terabytes of video archives that are accessed once a year for audit purposes. Moving these objects to a cold storage tier (Azure Archive, S3 Glacier, or Google Nearline) costs a fraction of hot storage. Questions like this test whether you understand storage tiers, access frequency tradeoffs, and retrieval latency requirements.
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: Ingress allow from 0.0.0.0/0 to web, ingress allow from web subnets to app, ingress allow from app subnets to db — Option B is correct because it follows the principle of least privilege for a multi-tier VPC. The web tier must be accessible from the internet (0.0.0.0/0) on ingress, the app tier must only accept ingress from the web tier subnets, and the db tier must only accept ingress from the app tier subnets. This ensures that each tier is isolated and only reachable from the immediate upstream tier, which is a fundamental security best practice for multi-tier architectures in Google Cloud.
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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Question Discussion
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