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Google ACE Practice Question: A network team is creating a new VPC and must…
A network team is creating a new VPC and must decide between auto mode and custom mode. Why would they choose custom mode?
⚠ Common exam trap
Google Cloud often tests the misconception that auto mode VPCs are more expensive or have IP limitations, when in fact the key differentiator is control over subnet placement and CIDR range, not cost or capacity.
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
✓
Custom mode allows full control over which regions have subnets and what CIDR ranges are used
Custom mode VPCs give the network team full control over the IP address range (CIDR block) and the ability to create subnets in any region, unlike auto mode VPCs which automatically create subnets in every region with a fixed /20 range per region. This is essential when you need to avoid overlapping CIDRs with on-premises networks or other VPCs, or when you want to restrict subnets to specific regions for compliance or cost reasons.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Auto mode VPCs cost more per subnet than custom mode
Why it's wrong here
Google Cloud does not charge for the number of subnets or the VPC mode; networking costs are based on data transfer, peering, and additional services like Cloud NAT or VPN. Auto mode may create more subnets (one per region) than a typical custom mode setup, but each subnet itself is free. Whether you choose auto or custom mode, the per-usage pricing is identical, so a larger number of subnets in auto mode does not result in higher VPC-related charges.
- ✓
Custom mode allows full control over which regions have subnets and what CIDR ranges are used
Why this is correct
In custom mode, you explicitly define every subnet, choosing the exact region and CIDR block, which lets you align your VPC address space with on-premises networks and avoid overlapping IP ranges that would break VPN or Interconnect peering. Unlike auto mode, which automatically creates subnets in all regions using a reserved 10.128.0.0/9 range (each with a /20), custom mode gives you the flexibility to create subnets only where needed and with appropriate sizes, preventing subnet sprawl and preserving address space for future growth. This control is essential for hybrid cloud designs that require careful CIDR planning to ensure route propagation doesn't conflict.
- ✗
Auto mode VPCs cannot be used with GKE clusters
Why it's wrong here
GKE clusters can be deployed in both auto mode and custom mode VPCs; the VPC's subnet mode does not impact GKE compatibility. What matters is whether the cluster uses VPC-native (alias IP) or routes-based networking, which is configured independently of VPC mode. In fact, auto mode VPCs provide a subnet in every region, which can simplify GKE cluster deployment since a subnet is always available wherever you create the cluster. The only real constraint is that the subnet used by the cluster must have sufficient available IP addresses for nodes and pods, not whether the VPC is auto or custom.
- ✗
Custom mode VPCs support more IP addresses per subnet than auto mode
Why it's wrong here
In both auto and custom modes, the size of a subnet is determined solely by its CIDR prefix length, and both support the same range from /8 to /29. Auto mode subnets are simply assigned a /20 by default, but they can be expanded to larger CIDR blocks using the same limits as custom mode. There is no architectural difference in IP capacity per subnet; the VPC mode only controls whether subnets are auto-created and how the address space is managed, not the maximum size of any individual subnet.
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GCP Cost Management
Key term
VPC
A Virtual Private Cloud (VPC) is a logically isolated section of a cloud provider's network where you can launch and manage resources like servers and databases with complete control over IP addressing, subnets, route tables, and security.
Key term
Region
A region is a distinct geographic location where a cloud provider operates multiple data centers that are connected by low-latency networks and provide cloud services.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This ACE 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 ACE exam.