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

An enterprise team is designing a custom-mode Virtual Private Cloud (VPC) network in Google Cloud that connects to an on-premises network via Cloud VPN. The on-premises network uses the IPv4 CIDR block 10.20.0.0/1610.20.0.0/16. The team has already configured a primary subnet in `us-east1` using 10.10.1.0/2410.10.1.0/24. They now need to plan a new primary subnet range for `us-west1` that must accommodate at least 10001000 Compute Engine instances while preventing any IP address space collisions with existing GCP subnets or on-premises networks. Which IPv4 CIDR block should the team allocate for the `us-west1` subnet?

  1. 10.10.4.0/2210.10.4.0/22Answer
  2. B
    10.10.0.0/2210.10.0.0/22
  3. C
    10.20.4.0/2210.20.4.0/22
  4. D
    10.10.2.0/2410.10.2.0/24

Answer

Allocating 10.10.4.0/2210.10.4.0/22 fulfills the capacity requirement of at least 10001000 instances without overlapping existing GCP subnets or on-premises networks.
The CIDR block 10.10.4.0/2210.10.4.0/22 provides 10241024 total IP addresses (10191019 usable), which easily accommodates 10001000 instances. Furthermore, its address range (10.10.4.010.10.4.0 through 10.10.7.25510.10.7.255) avoids any overlap with the existing 10.10.1.0/2410.10.1.0/24 subnet in `us-east1` and the 10.20.0.0/1610.20.0.0/16 on-premises network.

Step-by-Step Solution

1
Determine the minimum required CIDR block size for 10001000 instances.
A /24/24 subnet provides 256256 addresses, whereas a /22/22 subnet provides 20=10242^0 = 1024 addresses, meeting the requirement of at least 10001000 instances.
Google Cloud reserves 44 IP addresses per subnet (55 total unusable for arbitrary host assignment), making a /22/22 range necessary to host 10001000 VMs.
2
Identify existing reserved CIDR blocks to avoid IP overlap.
`us-east1` uses 10.10.1.0/2410.10.1.0/24 (10.10.1.010.10.1.0 - 10.10.1.25510.10.1.255) and on-premises uses 10.20.0.0/1610.20.0.0/16 (10.20.0.010.20.0.0 - 10.20.255.25510.20.255.255).
Subnet ranges within a VPC and across VPN-connected hybrid networks must be completely non-overlapping.
3
Evaluate the valid CIDR range options for non-overlapping boundaries.
The range 10.10.4.0/2210.10.4.0/22 spans 10.10.4.010.10.4.0 to 10.10.7.25510.10.7.255, which is entirely contiguous and clear of both 10.10.1.0/2410.10.1.0/24 and 10.20.0.0/1610.20.0.0/16.
It maintains non-overlapping primary IP addresses while fulfilling capacity limits.

Key Concept

VPC Subnet IP Address Planning and Overlap Avoidance
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Topics

Planning Virtual Private Cloud (VPC) Networks and Subnets

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