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.

Practice

1. What happens when you create a VPC network in auto subnet mode in GCP?

easy

A. Subnets are automatically created in all regions with predefined IP ranges.

B. No subnets are created; you must create them manually.

C. Only one subnet is created in the default region.

D. Subnets are created but without any IP ranges assigned.

Solution

1. Step 1: Understand auto subnet mode behavior

   In auto mode, GCP automatically creates subnets in every region with default IP ranges.

2. Step 2: Compare with other modes

   Unlike custom mode, auto mode does not require manual subnet creation or IP range assignment.

3. Final Answer:

   Subnets are automatically created in all regions with predefined IP ranges. -> Option A

4. Quick Check:

   Auto mode = automatic subnet creation [OK]

Hint: Auto mode means automatic subnets in all regions [OK]

Common Mistakes:

• Thinking auto mode requires manual subnet creation
• Assuming only one subnet is created
• Believing subnets have no IP ranges assigned

2. Which of the following is the correct way to create a custom subnet in GCP using gcloud CLI?

easy

A. gcloud compute networks subnets create my-network --subnet-mode=custom

B. gcloud compute networks create my-network --subnet-mode=auto

C. gcloud compute networks create my-subnet --range=10.0.0.0/24

D. gcloud compute networks subnets create my-subnet --network=my-network --range=10.0.0.0/24

Solution

1. Step 1: Identify command to create a subnet

   The command to create a subnet is gcloud compute networks subnets create with network and IP range specified.

2. Step 2: Check option correctness

   gcloud compute networks subnets create my-subnet --network=my-network --range=10.0.0.0/24 correctly uses the subnet creation command with network and IP range parameters.

3. Final Answer:

   gcloud compute networks subnets create my-subnet --network=my-network --range=10.0.0.0/24 -> Option D

4. Quick Check:

   Subnet creation uses 'networks subnets create' with range [OK]

Hint: Use 'networks subnets create' with --range for custom subnets [OK]

Common Mistakes:

• Using 'networks create' to create subnets
• Missing the --range parameter for IP range
• Confusing network and subnet names

3. Given a VPC network in custom subnet mode with two subnets:
subnet-a: 10.1.0.0/16
subnet-b: 10.2.0.0/16
What happens if you try to create a third subnet with IP range 10.1.128.0/17?

medium

A. The subnet is created successfully without issues.

B. The subnet is created but traffic is blocked between subnets.

C. Creation fails due to overlapping IP ranges with subnet-a.

D. The subnet is created but assigned a different IP range automatically.

Solution

1. Step 1: Check IP range overlap

   Subnet-a uses 10.1.0.0/16 which covers 10.1.0.0 to 10.1.255.255. The new subnet 10.1.128.0/17 overlaps this range.

2. Step 2: Understand subnet creation rules

   GCP does not allow overlapping IP ranges in subnets within the same VPC network.

3. Final Answer:

   Creation fails due to overlapping IP ranges with subnet-a. -> Option C

4. Quick Check:

   Overlapping IP ranges cause subnet creation failure [OK]

Hint: Check IP ranges for overlap before subnet creation [OK]

Common Mistakes:

• Assuming subnets can overlap IP ranges
• Thinking GCP auto-adjusts overlapping ranges
• Believing traffic is blocked but subnet created

4. You created a VPC network in custom subnet mode but forgot to create any subnets. What is the result when you try to deploy a VM instance in this network?

medium

A. The VM instance deploys successfully with an automatic subnet created.

B. The VM deployment fails because no subnet exists in the network.

C. The VM deploys but without an internal IP address.

D. The VM deploys but is not reachable from other resources.

Solution

1. Step 1: Understand custom subnet mode requirements

   In custom mode, subnets must be created manually before deploying resources.

2. Step 2: Check VM deployment dependency

   VMs require a subnet to get an IP address; without subnets, deployment fails.

3. Final Answer:

   The VM deployment fails because no subnet exists in the network. -> Option B

4. Quick Check:

   Custom mode needs subnets before VM deployment [OK]

Hint: No subnet means VM deployment fails in custom mode [OK]

Common Mistakes:

• Assuming auto subnet creation in custom mode
• Thinking VM can deploy without internal IP
• Believing VM deploys but is unreachable

5. You want to create a VPC network that spans multiple regions with subnets having specific IP ranges you control. Which subnet mode should you choose and why?

hard

A. Custom mode, because it lets you manually create subnets with specific IP ranges in each region.

B. Custom mode, because it automatically creates subnets in all regions with default IP ranges.

C. Auto mode, because it allows you to edit IP ranges after subnet creation.

D. Auto mode, because it creates subnets automatically with your chosen IP ranges.

Solution

1. Step 1: Identify requirement for specific IP ranges

   You want control over IP ranges, so automatic default ranges won't work.

2. Step 2: Choose subnet mode matching control needs

   Custom mode allows manual subnet creation with chosen IP ranges per region.

3. Step 3: Eliminate incorrect options

   Auto mode does not allow choosing IP ranges; it creates default subnets automatically.

4. Final Answer:

   Custom mode, because it lets you manually create subnets with specific IP ranges in each region. -> Option A

5. Quick Check:

   Custom mode = manual subnet creation with chosen IP ranges [OK]

Hint: Custom mode for manual subnets with specific IP ranges [OK]

Common Mistakes:

• Confusing auto mode as allowing custom IP ranges
• Thinking auto mode subnets can be edited after creation
• Believing custom mode auto-creates subnets