Question

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

An enterprise architecture team is configuring a custom-mode Virtual Private Cloud (VPC) network connected to two on-premises data centers via Cloud VPN.

The current network IP address allocations are:
- Data Center A on-premises range: 10.140.0.0/2110.140.0.0/21
- Data Center B on-premises range: 10.140.16.0/2010.140.16.0/20
- Existing GCP subnet primary range in `europe-west1`: 10.140.8.0/2210.140.8.0/22

To accommodate growing compute requirements without creating a new subnet, you must expand the primary IP address range of the `europe-west1` subnet to double its host address capacity. The expanded range must encompass the existing subnet without causing IP address overlap with either on-premises data center.

Which CIDR block must you specify for the expanded primary subnet range?

  1. 10.140.8.0/2110.140.8.0/21Answer
  2. B
    10.140.0.0/2110.140.0.0/21
  3. C
    10.140.0.0/2010.140.0.0/20
  4. D
    10.140.8.0/2010.140.8.0/20

Answer

The correct subnet expansion CIDR is 10.140.8.0/2110.140.8.0/21.
In Google Cloud VPC subnet design, expanding a primary IP range requires reducing the netmask prefix length (e.g., from /22/22 to /21/21) while ensuring the new block completely contains the existing range. The range 10.140.8.0/2110.140.8.0/21 encompasses all IPs from 10.140.8.010.140.8.0 through 10.140.15.25510.140.15.255, covering the original 10.140.8.0/2210.140.8.0/22 block (10.140.8.010.140.11.25510.140.8.0 - 10.140.11.255) without colliding with Data Center A (10.140.0.0/2110.140.0.0/21) or Data Center B (10.140.16.0/2010.140.16.0/20).

Step-by-Step Solution

1
Analyze the current GCP primary subnet range and address boundaries.
The existing subnet 10.140.8.0/2210.140.8.0/22 spans IP range 10.140.8.010.140.8.0 to 10.140.11.25510.140.11.255 (1,0241,024 addresses).
GCP rules mandate that any expanded subnet CIDR must fully encompass the current primary CIDR block.
2
Calculate the required prefix length to double host capacity.
Decreasing the prefix length by 1 bit (from /22/22 to /21/21) doubles the capacity to 2,0482,048 addresses.
A /21/21 block contains 211=2,0482^{11} = 2,048 addresses.
3
Determine valid /21/21 supernets that contain 10.140.8.0/2210.140.8.0/22.
The block 10.140.8.0/2110.140.8.0/21 spans 10.140.8.010.140.8.0 to 10.140.15.25510.140.15.255, which encompasses 10.140.8.0/2210.140.8.0/22.
Subnet boundaries for /21/21 prefixes align on multiples of 8 in the third octet.
4
Verify non-overlap constraints against on-premises environments.
10.140.8.0/2110.140.8.0/21 (10.140.8.010.140.15.25510.140.8.0 - 10.140.15.255) sits cleanly between Data Center A (10.140.0.010.140.7.25510.140.0.0 - 10.140.7.255) and Data Center B (10.140.16.010.140.31.25510.140.16.0 - 10.140.31.255).
Hybrid Cloud VPN connections require strictly non-overlapping IP address spaces across all connected environments.

Key Concept

VPC Subnet CIDR Range Expansion Rules and Hybrid Non-Overlap Requirements
Estimated Time:2m 0s
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