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Difficulty: HardPlanning Virtual Private Cloud (VPC) Networks and Subnets

An organization operates a custom-mode Virtual Private Cloud (VPC) network connected to an on-premises data center. A critical application deployed in the `us-east4` region resides within a primary subnet using the IP range 172.16.12.0/22172.16.12.0/22. Due to an influx of new microservice instances, the cloud engineering team needs to double the IP capacity of this primary subnet without recreating the subnet, causing service downtime, or altering the IP addresses of running Virtual Machines (VMs). Which primary subnet expansion configuration meets these requirements?

  1. Expand the primary IP address range of the existing subnet to 172.16.8.0/21172.16.8.0/21.Answer
  2. B
    Expand the primary IP address range of the existing subnet to 172.16.12.0/21172.16.12.0/21.
  3. C
    Add a secondary IP range of 172.16.16.0/22172.16.16.0/22 to the subnet and assign it as the primary interface network for new VM deployments.
  4. D
    Convert the VPC network from custom mode to auto mode to allow Google Cloud to automatically resize the subnet.

Answer

Expand the primary IP address range of the existing subnet to 172.16.8.0/21172.16.8.0/21.
Expanding the primary IP address range to 172.16.8.0/21172.16.8.0/21 adheres to Google Cloud's subnet expansion rules. A /21/21 subnet contains 2,048 addresses spanning 172.16.8.0172.16.8.0 to 172.16.15.255172.16.15.255. Because this range fully includes the original 172.16.12.0/22172.16.12.0/22 range (172.16.12.0172.16.12.0 to 172.16.15.255172.16.15.255) and starts on a valid /21/21 CIDR boundary (172.16.8.0172.16.8.0), the expansion can be performed online without modifying existing instance IPs.

Step-by-Step Solution

1
Analyze the existing primary subnet CIDR block and capacity requirements.
The current primary range is 172.16.12.0/22172.16.12.0/22, spanning host addresses 172.16.12.0172.16.12.0 through 172.16.15.255172.16.15.255 (1,024 total IP addresses). Doubling capacity requires a /21/21 prefix (2,048 total IP addresses).
Expanding a primary subnet in GCP VPC requires decreasing the prefix length (e.g., from /22/22 to /21/21) while keeping existing IP addresses intact.
2
Identify valid /21/21 CIDR block boundaries that encompass 172.16.12.0/22172.16.12.0/22.
A /21/21 block spans 8 contiguous /24/24 subnets (e.g., 172.16.0.0/21172.16.0.0/21 covers 0.0−7.2550.0-7.255, and 172.16.8.0/21172.16.8.0/21 covers 8.0−15.2558.0-15.255). The block 172.16.8.0/21172.16.8.0/21 contains the original 172.16.12.0/22172.16.12.0/22 range.
GCP subnet expansion rules mandate that the new range must align with standard network boundaries and completely encompass the existing primary IP address range.
3
Evaluate compliance with non-disruptive expansion guidelines.
Expanding the subnet range to 172.16.8.0/21172.16.8.0/21 requires no instance recreation, causes no downtime, and retains existing VM IP addresses.
In-place subnet range expansion in Google Cloud is an online operation as long as the new prefix is larger (smaller netmask) and encompasses the original block.

Key Concept

GCP VPC Primary Subnet Expansion CIDR Boundary Alignment
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Topics

Planning Virtual Private Cloud (VPC) Networks and Subnets

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