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

An organization is designing a custom mode Virtual Private Cloud (VPC) network in Google Cloud that connects to an on-premises data center via Cloud VPN. The on-premises network utilizes the IP address block 10.100.0.0/1610.100.0.0/16. Within the GCP VPC network, an engineer creates a primary subnet named `prod-subnet-us` in the `us-central1` region with an initial CIDR range of 10.1.0.0/2010.1.0.0/20. To accommodate incoming application workloads, the team needs to expand the primary IP range of `prod-subnet-us`. Which of the following expansion plans complies with Google Cloud VPC subnet expansion rules without causing IP range overlaps or routing conflicts with the on-premises network?

  1. Expand the primary IP range of `prod-subnet-us` to 10.1.0.0/1910.1.0.0/19.Answer
  2. B
    Expand the primary IP range of `prod-subnet-us` to 10.100.0.0/1510.100.0.0/15 to cover both cloud and on-premises systems.
  3. C
    Decrease the primary IP range of `prod-subnet-us` to 10.1.0.0/2410.1.0.0/24 and create a second non-contiguous primary subnet in `us-central1` with 10.1.1.0/2410.1.1.0/24.
  4. D
    Convert the VPC network from custom mode to auto mode so Google Cloud can automatically expand the subnet without IP collisions.

Answer

Expanding the primary IP range of `prod-subnet-us` to 10.1.0.0/1910.1.0.0/19 is the correct choice.
Expanding the primary subnet range to 10.1.0.0/1910.1.0.0/19 follows GCP subnet expansion rules: the prefix length decreases (from /20/20 to /19/19), the new range (10.1.0.010.1.0.0 - 10.1.31.25510.1.31.255) strictly encloses the existing range (10.1.0.010.1.0.0 - 10.1.15.25510.1.15.255), and it does not overlap with the on-premises range 10.100.0.0/1610.100.0.0/16.

Step-by-Step Solution

1
Analyze Google Cloud subnet expansion rules
In GCP, expanding a primary IP range requires choosing a smaller netmask prefix (e.g., from /20/20 to /19/19) such that the new range completely encloses the original range.
Google Cloud subnets cannot be shrunk, nor can their starting IP address be arbitrarily changed outside valid CIDR supernet alignment.
2
Calculate CIDR boundary for the expanded range 10.1.0.0/1910.1.0.0/19
The range 10.1.0.0/1910.1.0.0/19 spans 10.1.0.010.1.0.0 through 10.1.31.25510.1.31.255, which fully encompasses 10.1.0.0/2010.1.0.0/20 (10.1.0.010.1.0.0 through 10.1.15.25510.1.15.255).
This verifies technical compliance with GCP VPC primary subnet expansion criteria.
3
Verify hybrid network non-overlap requirement
The on-premises network uses 10.100.0.0/1610.100.0.0/16 (10.100.0.010.100.0.0 - 10.100.255.25510.100.255.255). The expanded GCP range 10.1.0.0/1910.1.0.0/19 has no overlap with 10.100.0.0/1610.100.0.0/16.
Preventing overlapping CIDR blocks between GCP VPCs and on-premises networks connected via VPN is necessary for proper BGP routing and traffic delivery.

Key Concept

Subnet IP Range Expansion and Overlap Prevention in Custom Mode VPCs
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Topics

Planning Virtual Private Cloud (VPC) Networks and Subnets

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