Subnet modes (auto, custom) in GCP - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding subnet creation in auto mode

In Google Cloud Platform, when you create a VPC network in auto subnet mode, what happens automatically?

A No subnets are created; the network remains empty until you add subnets.

B Subnets are created automatically in each region with predefined IP ranges.

C Subnets are created only in the region where the VPC was created.

D You must manually create subnets in each region after creating the VPC.

💡 Hint

Think about what 'auto' means for subnet creation across regions.

In auto subnet mode, Google Cloud automatically creates one subnet per region with predefined IP ranges. This simplifies network setup across all regions.

❓ Architecture

intermediate

Choosing subnet mode for custom IP ranges

You want to create a VPC network where you control the IP ranges of subnets in each region. Which subnet mode should you choose?

A Custom subnet mode

B Shared VPC mode

C Legacy network mode

D Auto subnet mode

💡 Hint

Consider which mode allows manual subnet creation with custom IP ranges.

Custom subnet mode lets you create subnets manually and specify IP ranges for each subnet in each region.

❓ service_behavior

advanced

Behavior of auto subnet mode when adding a new region

If you have a VPC network in auto subnet mode and Google Cloud adds a new region, what happens to your VPC network?

A A new subnet is automatically created in the new region with a predefined IP range.

B You must manually create a subnet in the new region to use it.

C The VPC network stops working until you update it for the new region.

D The new region is not available for use with the existing VPC network.

💡 Hint

Think about how auto subnet mode manages regions automatically.

Auto subnet mode automatically creates subnets in new regions added by Google Cloud, ensuring the network covers all regions.

❓ security

advanced

Security implications of auto vs custom subnet modes

Which subnet mode provides better control over network segmentation and security policies in Google Cloud?

A Auto subnet mode, because subnets are predefined and managed by Google.

B Auto subnet mode, because it automatically updates subnets in all regions.

C Custom subnet mode, because you can define subnets and IP ranges tailored to your security needs.

D Both modes provide the same level of control over network segmentation.

💡 Hint

Consider which mode lets you customize IP ranges and subnet boundaries.

Custom subnet mode allows you to define subnet IP ranges and boundaries, enabling precise network segmentation and security policy application.

✅ Best Practice

expert

Best practice for migrating from auto to custom subnet mode

You have an existing VPC network in auto subnet mode but want to migrate to custom subnet mode to control IP ranges. What is the best practice to achieve this migration?

A Use a command to convert the VPC network mode without downtime.

B Change the existing VPC network's mode directly from auto to custom in the console.

C Delete all auto-created subnets and then add custom subnets in the same VPC.

D Create a new VPC network in custom mode and migrate resources to it gradually.

💡 Hint

Consider if subnet mode can be changed directly on an existing VPC.

Subnet mode cannot be changed after creation. The best practice is to create a new VPC in custom mode and migrate resources carefully.

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