The answer is to create three subnets: 10.0.1.0/24 for web, 10.0.2.0/23 for app, and 10.0.4.0/26 for db. This subnet IP planning for multi-tier applications is correct because it matches each tier’s instance count to a subnet size that provides exactly the needed IPs plus room for growth—a /24 offers 256 addresses for 100 web instances, a /23 offers 512 for 200 app instances, and a /26 offers 64 for 50 db instances—minimizing wasted address space while keeping each tier isolated for granular firewall rules and routing. On the Google Professional Cloud Network Engineer exam, this tests your ability to balance cost-efficiency with scalability in custom VPC design; a common trap is choosing a single large subnet or overlapping ranges, which wastes IPs and complicates security. Remember the memory tip: “Web gets a /24, App gets a /23, DB gets a /26—match the mask to the max, and you’ll never be lax.”
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 network engineer is designing a VPC with custom subnet mode. They need to allocate IP addresses for three tiers: web (100 instances), app (200 instances), and db (50 instances). The VPC will be in the us-central1 region. Which subnet plan is most cost-effective and scalable?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Create three subnets: 10.0.1.0/24 (web), 10.0.2.0/23 (app), 10.0.4.0/26 (db)
Option D is correct because it allocates IP addresses efficiently for each tier: a /24 (256 IPs) for web (100 instances), a /23 (512 IPs) for app (200 instances), and a /26 (64 IPs) for db (50 instances). This minimizes wasted IP space while providing room for growth, and using separate subnets per tier allows granular firewall rules and routing. In a custom VPC, this design is both cost-effective (no over-provisioning) and scalable (each tier can expand within its subnet).
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.
✗
Use one subnet per zone: us-central1-a: 10.0.0.0/22, us-central1-b: 10.0.4.0/22, us-central1-c: 10.0.8.0/22
Why it's wrong here
Overly large and not tier-specific; wastes IP addresses and is more complex.
✗
Create two subnets: 10.0.1.0/24 (web+app) and 10.0.3.0/24 (db)
Why it's wrong here
Combining web and app into one subnet is not scalable and may introduce security risks.
✗
Create one subnet 10.0.0.0/20 and use network tags to isolate tiers via firewall rules
Why it's wrong here
While possible, it reduces network segmentation and complicates routing.
✓
Create three subnets: 10.0.1.0/24 (web), 10.0.2.0/23 (app), 10.0.4.0/26 (db)
Why this is correct
Adequate sizes, separate subnets provide security and flexibility.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the misconception that larger subnets are always better for scalability, but the trap here is that over-provisioning IPs (e.g., /22 or /20) wastes address space and can lead to higher costs or management overhead, whereas right-sizing subnets per tier with room for growth is the most cost-effective and scalable approach.
Detailed technical explanation
How to think about this question
In GCP, subnet sizes are defined by CIDR blocks, and each subnet's usable IP count is the total minus the reserved addresses (network, gateway, and broadcast, though GCP reserves four per subnet). A /26 provides 64 IPs (60 usable), which is sufficient for 50 db instances with some headroom. The /23 for app (512 IPs, ~508 usable) allows for future scaling beyond 200 instances, while the /24 for web (256 IPs, ~252 usable) comfortably fits 100 instances. This design follows the principle of right-sizing subnets to avoid IP waste, which directly impacts cost in terms of reserved IPs and potential need for renumbering.
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 startup's cloud architect reviews their monthly bill and notices costs are higher than expected for a long-running batch job. Switching from on-demand instances to Reserved Instances — or using Spot/Preemptible VMs — can reduce compute costs by up to 72 %. Questions like this test whether you understand the tradeoffs between commitment, flexibility, and cost across cloud pricing models.
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: Create three subnets: 10.0.1.0/24 (web), 10.0.2.0/23 (app), 10.0.4.0/26 (db) — Option D is correct because it allocates IP addresses efficiently for each tier: a /24 (256 IPs) for web (100 instances), a /23 (512 IPs) for app (200 instances), and a /26 (64 IPs) for db (50 instances). This minimizes wasted IP space while providing room for growth, and using separate subnets per tier allows granular firewall rules and routing. In a custom VPC, this design is both cost-effective (no over-provisioning) and scalable (each tier can expand within its subnet).
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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