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Difficulty: Very hardPlanning Virtual Private Cloud (VPC) Networks and Subnets

An enterprise architecture team is configuring a custom-mode Virtual Private Cloud (VPC) network in Google Cloud. The primary subnet in `us-east4` is currently assigned the IPv4 CIDR block 10.240.0.0/2210.240.0.0/22. The team plans to deploy a Google Kubernetes Engine (GKE) cluster in this subnet and must define a secondary IPv4 CIDR range for Pod IP address allocation. Furthermore, long-term capacity requirements mandate that the primary subnet range must be expandable to 10.240.0.0/2010.240.0.0/20 in the future without recreating the subnet, modifying existing secondary ranges, or creating IP range overlaps. Which secondary IPv4 CIDR range should be assigned to the Pods to satisfy all requirements?

  1. 10.240.16.0/2110.240.16.0/21Answer
  2. B
    10.240.4.0/2210.240.4.0/22
  3. C
    10.240.8.0/2110.240.8.0/21
  4. D
    10.240.2.0/2310.240.2.0/23

Answer

Assigning the secondary Pod range to 10.240.16.0/2110.240.16.0/21 ensures that the primary IP range can be seamlessly expanded to 10.240.0.0/2010.240.0.0/20 without IP overlap.
The range 10.240.16.0/2110.240.16.0/21 spans from 10.240.16.010.240.16.0 to 10.240.23.25510.240.23.255. Because expanding the primary subnet from 10.240.0.0/2210.240.0.0/22 to 10.240.0.0/2010.240.0.0/20 encompasses the IP span from 10.240.0.010.240.0.0 to 10.240.15.25510.240.15.255, placing the secondary range at 10.240.16.0/2110.240.16.0/21 leaves the entire 10.240.0.0/2010.240.0.0/20 block unobstructed for future expansion.

Step-by-Step Solution

1
Calculate the IP range boundaries for the target expanded primary subnet.
Expanding 10.240.0.0/2210.240.0.0/22 to 10.240.0.0/2010.240.0.0/20 requires a contiguous block spanning from 10.240.0.010.240.0.0 to 10.240.15.25510.240.15.255.
GCP subnet primary expansion requires lowering the prefix length (e.g., /22 to /20) while maintaining the original network address alignment.
2
Evaluate secondary IP range constraints against GCP VPC networking rules.
A subnet's secondary IP ranges cannot overlap with its own primary IP range, nor can they overlap with any future space reserved for primary IP range expansion.
If a secondary IP range exists within 10.240.0.0/2010.240.0.0/20, GCP will reject any subsequent request to expand the primary range into that overlapping space.
3
Identify the CIDR block located entirely outside the expanded 10.240.0.0/2010.240.0.0/20 space.
The range 10.240.16.0/2110.240.16.0/21 covers 10.240.16.010.240.16.0 to 10.240.23.25510.240.23.255, which starts immediately after 10.240.15.25510.240.15.255 and provides zero overlap.
This guarantees that both current operations and future primary CIDR expansion can proceed without modifying or recreating the secondary range.

Key Concept

VPC Subnet Primary Expansion and Secondary IP Range Non-Overlap Rules
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Topics

Planning Virtual Private Cloud (VPC) Networks and Subnets

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