Quick Answer
The answer is auto mode subnet. This is the correct choice because an auto mode VPC automatically creates a subnet in every Google Cloud region, using a predefined IP range from the 10.0.0.0/8 pool that can expand as you add more VM instances, allowing the network to grow without manual reconfiguration. In contrast, a custom mode subnet requires you to manually define each subnet and its CIDR block, offering more control but no automatic scaling. On the Google Professional Cloud Network Engineer exam, this question tests your understanding of VPC subnet types and their use cases, often appearing as a scenario where a startup needs dynamic, hands-off growth. A common trap is assuming custom mode is always better for control, but auto mode is specifically designed for environments that prioritize automatic expansion. Memory tip: think “Auto = Automatic growth across all regions,” like a self-expanding network that saves you from manual subnet planning.
This PCNE practice question tests your understanding of implementing a virtual private cloud. 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 startup wants to create a VPC with a subnet that can grow automatically as they add more VM instances. Which subnet type should they use?
Custom mode subnet
Why wrong: Custom mode gives full control but requires manual range expansion.
Dynamic subnet
Why wrong: No such subnet mode exists in Google Cloud.
Legacy network
Why wrong: Legacy networks are deprecated and not recommended.
Auto mode subnet
Auto mode subnets automatically allocate IP ranges and expand as needed.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Auto mode subnet
An auto mode subnet automatically creates subnets in each region and assigns IP address ranges from a predefined pool that can expand as you add more VM instances. This allows the subnet to grow without manual intervention, making it ideal for startups that need dynamic scaling.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
Custom mode subnet
Why it's wrong here
Custom mode gives full control but requires manual range expansion.
Dynamic subnet
Why it's wrong here
No such subnet mode exists in Google Cloud.
Legacy network
Why it's wrong here
Legacy networks are deprecated and not recommended.
Auto mode subnet
Why this is correct
Auto mode subnets automatically allocate IP ranges and expand as needed.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Google Cloud often tests the distinction between auto mode and custom mode subnets, where candidates mistakenly think 'dynamic subnet' is a real option or assume custom mode can auto-expand, but only auto mode provides automatic regional subnet creation and growth.
Detailed technical explanation
Auto mode VPCs use a /20 subnet per region from the 10.128.0.0/9 range, allowing up to 16 regions with automatic IP allocation. When you add a VM without specifying a subnet, the system assigns it to the auto-created subnet in the same region, and if the subnet runs out of IPs, you can expand it by adding secondary CIDR ranges (e.g., /24) without downtime. This contrasts with custom mode, where you must manually plan and allocate CIDR blocks, risking IP exhaustion if not carefully managed.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
An e-commerce site experiences heavy traffic on Black Friday and near-zero traffic during off-peak weeks. Rather than provisioning permanent large VMs, the team uses auto-scaling groups that add capacity automatically under load and reduce it overnight. Questions like this test whether you understand elasticity, availability zones, and cloud compute scaling patterns.
What to study next
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FAQ
Implementing a Virtual Private Cloud — This question tests Implementing a Virtual Private Cloud — Read the scenario before looking for a memorised answer..
The correct answer is: Auto mode subnet — An auto mode subnet automatically creates subnets in each region and assigns IP address ranges from a predefined pool that can expand as you add more VM instances. This allows the subnet to grow without manual intervention, making it ideal for startups that need dynamic scaling.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 30, 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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