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

A cloud architect is designing a custom-mode Virtual Private Cloud (VPC) network. The primary subnet in `us-east1` is currently configured with the IP block 10.200.1.0/2410.200.1.0/24. The architecture team intends to expand this primary subnet range in the future to a /23/23 prefix to accommodate up to 500500 virtual machine instances. A second subnet in `us-central1` must be provisioned immediately. To ensure that the future expansion of the `us-east1` subnet will not fail due to IP range overlap, which CIDR range should be allocated to the new `us-central1` subnet?

  1. A
    10.200.0.0/2410.200.0.0/24
  2. 10.200.2.0/2410.200.2.0/24Answer
  3. C
    10.200.1.0/2310.200.1.0/23
  4. D
    10.200.1.128/2310.200.1.128/23

Answer

The range 10.200.2.0/2410.200.2.0/24 should be allocated to the us-central1 subnet to avoid CIDR overlap when us-east1 is expanded to 10.200.0.0/2310.200.0.0/23.
In Google Cloud VPC networking, primary IP address ranges must be expanded to a larger subnet mask (smaller prefix length) while maintaining strict bit-boundary alignment. Expanding 10.200.1.0/2410.200.1.0/24 to a /23/23 prefix forces the expanded range to become 10.200.0.0/2310.200.0.0/23, spanning 10.200.0.010.200.0.0 to 10.200.1.25510.200.1.255. Therefore, assigning 10.200.2.0/2410.200.2.0/24 to the new subnet leaves the address space 10.200.0.010.200.0.0–10.200.1.25510.200.1.255 completely free for future expansion.

Step-by-Step Solution

1
Determine the binary CIDR boundary for expanding the existing subnet.
Expanding 10.200.1.0/2410.200.1.0/24 to a /23/23 prefix requires aligning to a power-of-two boundary. The enclosing /23/23 block is 10.200.0.0/2310.200.0.0/23, covering IP addresses 10.200.0.010.200.0.0 through 10.200.1.25510.200.1.255.
Google Cloud VPC subnets must align to strict bit-boundary boundaries when expanded.
2
Identify potential CIDR conflicts within the VPC network.
Any subnet allocated within the 10.200.0.0/2410.200.0.0/24 block will prevent the future expansion of 10.200.1.0/2410.200.1.0/24 because 10.200.0.0/2310.200.0.0/23 would overlap with it.
GCP VPC subnets within the same network cannot have overlapping primary IPv4 address ranges.
3
Select a non-overlapping contiguous /24/24 block for the new region.
The range 10.200.2.0/2410.200.2.0/24 starts immediately after the expanded 10.200.0.0/2310.200.0.0/23 range (10.200.0.010.200.0.0–10.200.1.25510.200.1.255), avoiding any overlap.
Choosing 10.200.2.0/2410.200.2.0/24 reserves the entire 10.200.0.0/2310.200.0.0/23 block for the us-east1 subnet expansion.

Key Concept

CIDR Subnet Expansion and Boundary Alignment in GCP VPC Networks
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Topics

Planning Virtual Private Cloud (VPC) Networks and Subnets

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