Custom VPC creation in GCP - Time & Space Complexity

Time Complexity: Custom VPC creation

O(n + m)

Understanding Time Complexity

When creating a custom Virtual Private Cloud (VPC) in GCP, it is important to understand how the time to complete the setup changes as you add more components.

We want to know how the number of steps or API calls grows when we create more subnets or firewall rules.

Scenario Under Consideration

Analyze the time complexity of the following operation sequence.

# Create a custom VPC network
gcloud compute networks create my-vpc --subnet-mode=custom

# Create multiple subnets in the VPC
for i in $(seq 1 n); do
  gcloud compute networks subnets create subnet-$i --network=my-vpc --region=region-$i --range=10.$i.0.0/24
done

# Create firewall rules for the VPC
for i in $(seq 1 m); do
  gcloud compute firewall-rules create fw-rule-$i --network=my-vpc --allow tcp:80
done

This sequence creates a custom VPC, then adds multiple subnets and firewall rules to it.

Identify Repeating Operations

Identify the API calls, resource provisioning, data transfers that repeat.

• Primary operation: Creating subnets and firewall rules via API calls.
• How many times: Subnet creation happens n times; firewall rule creation happens m times.

How Execution Grows With Input

Each subnet and firewall rule requires a separate API call, so the total number of calls grows as you add more.

Input Size (n + m)	Approx. Api Calls/Operations
10	About 10 calls (e.g., 5 subnets + 5 firewall rules)
100	About 100 calls (e.g., 50 subnets + 50 firewall rules)
1000	About 1000 calls (e.g., 500 subnets + 500 firewall rules)

Pattern observation: The total work grows directly with the number of subnets and firewall rules you create.

Final Time Complexity

Time Complexity: O(n + m)

This means the time to create the VPC setup grows linearly with the number of subnets and firewall rules added.

Common Mistake

[X] Wrong: "Creating one big VPC network means all subnets and rules are created instantly in one step."

[OK] Correct: Each subnet and firewall rule requires its own API call and setup time, so the total time grows with how many you add.

Interview Connect

Understanding how the number of resources affects setup time helps you plan and explain cloud infrastructure tasks clearly and confidently.

Self-Check

"What if we created all subnets in parallel instead of one after another? How would the time complexity change?"

Practice

1. What is the main advantage of creating a Custom VPC in Google Cloud Platform?

easy

A. You can define your own IP address ranges and subnets.

B. It automatically creates default firewall rules.

C. It provides free internet access without configuration.

D. It disables all network traffic by default.

Solution

1. Step 1: Understand Custom VPC purpose

   A Custom VPC allows you to design your network with your own IP ranges and subnets, unlike default VPCs which have preset ranges.

2. Step 2: Eliminate wrong options

   Custom VPC does not disable traffic, default firewall rules exist regardless, free internet requires configuration.

3. Final Answer:

   You can define your own IP address ranges and subnets. -> Option A

4. Quick Check:

   Custom VPC = Custom IP ranges [OK]

Hint: Custom VPC means you pick your IP ranges [OK]

Common Mistakes:

• Confusing default VPC with custom VPC
• Thinking firewall rules are auto-created
• Assuming internet access is automatic

2. Which gcloud command correctly creates a custom VPC named my-vpc with no automatic subnet creation?

easy

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

B. gcloud compute networks create my-vpc --auto-create-subnetworks

C. gcloud compute networks create my-vpc --no-subnet-mode

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

Solution

1. Step 1: Identify subnet mode for custom VPC

   Custom VPC requires the flag --subnet-mode=custom to avoid automatic subnet creation.

2. Step 2: Evaluate options

   --subnet-mode=auto creates automatic subnets. --auto-create-subnetworks uses invalid syntax. --no-subnet-mode does not exist.

3. Final Answer:

   gcloud compute networks create my-vpc --subnet-mode=custom -> Option D

4. Quick Check:

   Custom VPC uses --subnet-mode=custom [OK]

Hint: Use --subnet-mode=custom to create custom VPC [OK]

Common Mistakes:

• Using --subnet-mode=auto instead of custom
• Using invalid flags like --no-subnet-mode
• Assuming subnets are created automatically

3. Given this command:

gcloud compute networks subnets create subnet-1 --network=my-vpc --region=us-central1 --range=10.0.1.0/24

What is the CIDR range assigned to subnet-1?

medium

A. 10.0.0.0/16

B. 192.168.1.0/24

C. 10.0.1.0/24

D. 10.1.0.0/24

Solution

1. Step 1: Read the subnet creation command

   The command specifies --range=10.0.1.0/24 which sets the IP range for the subnet.

2. Step 2: Match the CIDR range

   10.0.1.0/24 matches the exact CIDR range given in the command.

3. Final Answer:

   10.0.1.0/24 -> Option C

4. Quick Check:

   Subnet range = 10.0.1.0/24 [OK]

Hint: Look for --range flag for subnet CIDR [OK]

Common Mistakes:

• Confusing VPC range with subnet range
• Picking wrong CIDR block from options
• Ignoring the --range parameter

4. You ran this command to create a subnet:

gcloud compute networks subnets create subnet-2 --network=my-vpc --region=us-east1 --range=10.0.1.0/24

But you get an error saying the IP range overlaps with an existing subnet. What is the likely cause?

medium

A. The subnet range 10.0.1.0/24 overlaps with another subnet in the same VPC.

B. The region us-east1 is invalid for subnet creation.

C. The network my-vpc does not exist.

D. The command is missing the --subnet-mode flag.

Solution

1. Step 1: Understand the error message

   The error about overlapping IP range means the subnet's CIDR block conflicts with an existing subnet in the same VPC.

2. Step 2: Check other options

   Region and network existence errors produce different messages; --subnet-mode is for network creation, not subnet.

3. Final Answer:

   The subnet range 10.0.1.0/24 overlaps with another subnet in the same VPC. -> Option A

4. Quick Check:

   Overlapping CIDR causes subnet creation error [OK]

Hint: Check subnet CIDR overlaps before creating [OK]

Common Mistakes:

• Assuming region is invalid without checking
• Confusing network creation flags with subnet flags
• Ignoring existing subnet CIDR ranges

5. You want to create a custom VPC named prod-vpc with two subnets:
- subnet-a in us-west1 with range 10.10.1.0/24
- subnet-b in us-east1 with range 10.10.2.0/24
Which sequence of gcloud commands correctly creates this setup?

hard

A. 1) gcloud compute networks create prod-vpc --subnet-mode=auto
2) gcloud compute networks subnets create subnet-a --network=prod-vpc --region=us-west1 --range=10.10.1.0/24
3) gcloud compute networks subnets create subnet-b --network=prod-vpc --region=us-east1 --range=10.10.2.0/24

B. 1) gcloud compute networks create prod-vpc --subnet-mode=custom
2) gcloud compute networks subnets create subnet-a --network=prod-vpc --region=us-west1 --range=10.10.1.0/24
3) gcloud compute networks subnets create subnet-b --network=prod-vpc --region=us-east1 --range=10.10.2.0/24

C. 1) gcloud compute networks create prod-vpc
2) gcloud compute networks subnets create subnet-a --network=prod-vpc --region=us-west1 --range=10.10.1.0/24
3) gcloud compute networks subnets create subnet-b --network=prod-vpc --region=us-east1 --range=10.10.2.0/24

D. 1) gcloud compute networks create prod-vpc --subnet-mode=custom
2) gcloud compute networks subnets create subnet-a --network=prod-vpc --region=us-west1 --range=10.10.2.0/24
3) gcloud compute networks subnets create subnet-b --network=prod-vpc --region=us-east1 --range=10.10.1.0/24

Solution

1. Step 1: Create the VPC with custom subnet mode

   The VPC must be created with --subnet-mode=custom to allow manual subnet creation.

2. Step 2: Create subnets with correct regions and CIDR ranges

   Subnets must be created with specified regions and matching CIDR ranges as per requirements.

3. Step 3: Verify order and correctness

   1) gcloud compute networks create prod-vpc --subnet-mode=custom 2) gcloud compute networks subnets create subnet-a --network=prod-vpc --region=us-west1 --range=10.10.1.0/24 3) gcloud compute networks subnets create subnet-b --network=prod-vpc --region=us-east1 --range=10.10.2.0/24 correctly creates the VPC first, then subnets with correct ranges and regions. 1) gcloud compute networks create prod-vpc --subnet-mode=auto 2) gcloud compute networks subnets create subnet-a --network=prod-vpc --region=us-west1 --range=10.10.1.0/24 3) gcloud compute networks subnets create subnet-b --network=prod-vpc --region=us-east1 --range=10.10.2.0/24 uses auto subnet mode which auto-creates subnets, conflicting with manual subnet creation. 1) gcloud compute networks create prod-vpc 2) gcloud compute networks subnets create subnet-a --network=prod-vpc --region=us-west1 --range=10.10.1.0/24 3) gcloud compute networks subnets create subnet-b --network=prod-vpc --region=us-east1 --range=10.10.2.0/24 misses subnet mode flag. 1) gcloud compute networks create prod-vpc --subnet-mode=custom 2) gcloud compute networks subnets create subnet-a --network=prod-vpc --region=us-west1 --range=10.10.2.0/24 3) gcloud compute networks subnets create subnet-b --network=prod-vpc --region=us-east1 --range=10.10.1.0/24 swaps CIDR ranges between subnets.

4. Final Answer:

   Correct sequence with custom subnet mode and matching subnet ranges -> Option B

5. Quick Check:

   Custom VPC + correct subnet ranges = 1) gcloud compute networks create prod-vpc --subnet-mode=custom 2) gcloud compute networks subnets create subnet-a --network=prod-vpc --region=us-west1 --range=10.10.1.0/24 3) gcloud compute networks subnets create subnet-b --network=prod-vpc --region=us-east1 --range=10.10.2.0/24 [OK]

Hint: Create VPC with --subnet-mode=custom before adding subnets [OK]

Common Mistakes:

• Using auto subnet mode when manual subnets needed
• Swapping subnet CIDR ranges by mistake
• Omitting --subnet-mode flag on VPC creation