An enterprise cloud network engineer is planning network expansion for a custom-mode Virtu… — Google Cloud Associate Cloud Engineer

An enterprise cloud network engineer is planning network expansion for a custom-mode Virtual Private Cloud (VPC) network on Google Cloud. An application subnet in region `us-east4` currently uses the primary IP range $10.240.16.0/24$ and must be expanded to support at least 500 virtual machine instances.

The VPC network and connected environments currently have the following IP address allocations:
- Subnet A (Application subnet to expand): $10.240.16.0/24$
- Subnet B (Database subnet in `us-east4`): $10.240.17.0/24$
- Subnet C (Analytics subnet in `us-central1`): $10.240.18.0/23$
- On-premises data center network (connected via Cloud VPN): $10.240.20.0/22$

Which strategy should the network engineer implement to meet the capacity requirement without violating Google Cloud VPC networking constraints?

Provision a new subnet using the non-overlapping range $10.240.24.0/23$ in `us-east4` and migrate the application workloads to the new subnet.Answer
B
Expand the existing primary subnet CIDR block from $10.240.16.0/24$ to $10.240.16.0/23$ using the gcloud compute networks subnets expand-ip-range command.
C
Convert the VPC network from custom-mode to auto-mode so Google Cloud automatically resizes the subnet to accommodate 500 instances.
D
Change the primary subnet CIDR block from $10.240.16.0/24$ to $10.240.16.0/22$ to bypass the collision with $10.240.17.0/24$ .

Answer

The network engineer must provision a new subnet using a non-overlapping range such as

10.240.24.0/23

in `us-east4` and migrate the application workloads.

Expanding a primary subnet range in GCP requires contiguous IP space. The current subnet

10.240.16.0/24

expanded to

/23

covers

10.240.16.0

10.240.17.255

. Because

10.240.17.0/24

is already allocated to Subnet B, in-place expansion is rejected by Google Cloud. Therefore, creating a new non-overlapping subnet (such as

10.240.24.0/23

) and migrating workloads is the only valid solution.

Step-by-Step Solution

Calculate required CIDR prefix length for 500 hosts

/23

subnet provides

2^{(32-23)} = 512

total IP addresses (507 usable in GCP), meeting the requirement for 500 instances.

Google Cloud reserves 4 IP addresses per subnet plus 1 broadcast address, so a

/24

(256 addresses) is insufficient, while a

/23

(512 addresses) provides enough usable IPs.

Evaluate in-place expansion of primary IP range

10.240.16.0/24

Expanding

10.240.16.0/24

/23

produces the range

10.240.16.0

–

10.240.17.255

Subnet expansion in GCP can only decrease the netmask prefix length (making the range larger) while keeping the start IP aligned. The new

/23

block incorporates

10.240.17.0/24

Check for CIDR collisions within the VPC and hybrid network

10.240.17.0/24

is already assigned to Subnet B in the same VPC. Thus, expanding Subnet A in-place causes an illegal CIDR overlap.

GCP VPC architecture strictly prohibits overlapping primary IP ranges between subnets within the same VPC or connected via VPC Peering / Cloud VPN.

Determine the valid alternative architecture

Select an unallocated block such as

10.240.24.0/23

that does not conflict with existing subnets (

10.240.16.0/24

10.240.17.0/24

10.240.18.0/23

) or on-premises ranges (

10.240.20.0/22

When contiguous IP expansion is blocked by adjacent subnets, creating a new disjoint subnet with non-overlapping IP space is the required GCP architectural solution.

Key Concept

VPC Subnet IP Range Expansion Constraints and Overlap Prevention

Question

Answer

Step-by-Step Solution

Key Concept

Common Mistakes

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