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

A cloud engineer is designing a custom-mode Virtual Private Cloud (VPC) network. The current setup includes a primary subnet in `us-central1` configured with a CIDR block of 10.10.1.0/2410.10.1.0/24 and another primary subnet in the same VPC configured with 10.10.2.0/2410.10.2.0/24. Due to anticipated growth, the engineer needs to double the IP address capacity of the 10.10.1.0/2410.10.1.0/24 subnet without re-creating the subnet or disrupting existing workloads. Which CIDR range configuration update is valid and prevents IP range overlap?

  1. Expand the subnet primary IP range to 10.10.0.0/2310.10.0.0/23.Answer
  2. B
    Expand the subnet primary IP range to 10.10.1.0/2310.10.1.0/23.
  3. C
    Change the VPC mode from custom-mode to auto-mode to allow Google Cloud to automatically expand subnets.
  4. D
    Add a non-contiguous secondary CIDR range of 10.10.3.0/2410.10.3.0/24 directly to the primary IP range of the subnet.

Answer

Expand the subnet primary IP range to 10.10.0.0/2310.10.0.0/23.
Expanding the primary CIDR block of a custom-mode subnet requires keeping the range contiguous and aligned to proper binary network boundaries. Expanding 10.10.1.0/2410.10.1.0/24 to 10.10.0.0/2310.10.0.0/23 covers IP addresses from 10.10.0.010.10.0.0 to 10.10.1.25510.10.1.255, incorporating the original range, doubling capacity to 512512 addresses, and avoiding collision with the existing 10.10.2.0/2410.10.2.0/24 subnet.

Step-by-Step Solution

1
Analyze the starting primary IP address range and target capacity expansion.
The subnet currently uses 10.10.1.0/2410.10.1.0/24 (256256 IP addresses). Doubling capacity requires a /23/23 prefix (512512 IP addresses).
Expanding a subnet primary CIDR block requires shifting to a smaller prefix length while remaining contiguous.
2
Evaluate valid /23/23 network boundaries that include 10.10.1.0/2410.10.1.0/24.
A /23/23 subnet boundary aligned on binary boundaries covers 10.10.0.010.10.0.0 to 10.10.1.25510.10.1.255 (represented as 10.10.0.0/2310.10.0.0/23).
10.10.1.0/2310.10.1.0/23 is not a valid network address base; the network address for this /23/23 block is 10.10.0.0/2310.10.0.0/23.
3
Verify there is no IP range overlap with other subnets in the VPC.
10.10.0.0/2310.10.0.0/23 covers 10.10.0.010.10.0.0–10.10.1.25510.10.1.255, which does not collide with the neighboring subnet at 10.10.2.0/2410.10.2.0/24.
Google Cloud VPC subnet expansion fails if the new expanded CIDR range overlaps with any pre-existing subnets in the network.

Key Concept

Subnet IP Range Expansion in Google Cloud VPC
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Topics

Planning Virtual Private Cloud (VPC) Networks and Subnets

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