Subnet modes (auto, custom) in GCP - Step-by-Step Execution

Process Flow - Subnet modes (auto, custom)

Start VPC Creation

↓

Choose Subnet Mode

↓

Auto Mode

↓

System creates

↓

default subnets

↓

Ready to use

↓

Deploy VPC

When creating a VPC, you pick a subnet mode: auto creates default subnets automatically, custom requires you to create subnets yourself before use.

Execution Sample

GCP

gcloud compute networks create my-vpc --subnet-mode=auto

# vs

gcloud compute networks create my-vpc --subnet-mode=custom

gcloud compute networks subnets create my-subnet --network=my-vpc --region=us-central1 --range=10.0.0.0/24

Shows creating a VPC with auto mode (default subnets created) vs custom mode (user creates subnets manually).

Process Table

Step	Action	Subnet Mode	Subnets Created	Result
1	Create VPC with --subnet-mode=auto	auto	Default subnets in all regions	VPC ready with subnets
2	Create VPC with --subnet-mode=custom	custom	No subnets created	VPC ready but no subnets
3	Create subnet manually in custom VPC	custom	User-defined subnet in specified region	Subnet ready for use
4	Try to deploy VM in custom VPC without subnet	custom	No subnet	Deployment fails - no subnet
5	Deploy VM in auto VPC	auto	Uses default subnet	Deployment succeeds

💡 Execution stops after VPC and subnets are created and ready for resource deployment.

Status Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5
VPC State	None	Created with default subnets	Created without subnets	Created with user subnet	No subnets - deployment fails	Created with default subnets
Subnets	None	Default subnets present	None	One user subnet created	None	Default subnets present
Deployment Status	None	Not started	Not started	Not started	Fails - no subnet	Succeeds - uses default subnet

Key Moments - 2 Insights

Why does deployment fail in custom mode before creating subnets?

Because in custom mode, no subnets exist by default (see execution_table step 4). You must create subnets manually before deploying resources.

Are subnets automatically created in custom mode?

No, subnets are only automatically created in auto mode (execution_table step 1). Custom mode requires manual subnet creation (step 3).

Visual Quiz - 3 Questions

Look at the execution table, what subnets exist immediately after creating a VPC with --subnet-mode=custom?

A No subnets

B Default subnets in all regions

C One subnet in us-central1

D Subnets created only in us-east1

💡 Hint

Check execution_table row 2 under 'Subnets Created'

At which step does the deployment fail due to missing subnet?

A Step 2

B Step 4

C Step 3

D Step 5

💡 Hint

Look at execution_table row 4 under 'Result'

If you want subnets created automatically, which subnet mode should you choose?

A custom

B manual

C auto

D dynamic

💡 Hint

Refer to concept_flow and execution_table step 1

Concept Snapshot

Subnet modes in GCP VPC:
- auto: system creates default subnets in all regions automatically
- custom: user creates subnets manually before use
- auto mode is quick for simple setups
- custom mode gives control over subnet ranges and regions
- resources need subnets to deploy; no subnets means deployment fails

Full Transcript

When you create a Virtual Private Cloud (VPC) network in Google Cloud Platform, you must choose a subnet mode. The two main modes are auto and custom. In auto mode, Google Cloud automatically creates default subnets in all regions for you. This means you can start deploying resources right away. In custom mode, no subnets are created automatically. You must create subnets yourself in the regions you want. If you try to deploy resources without creating subnets in custom mode, deployment will fail. Auto mode is easier for beginners or simple projects. Custom mode is better when you want full control over subnet IP ranges and locations. Always remember that subnets are required for resources to connect inside the VPC.