Subnets & IP Addressing

Subnets are the regional building blocks of a GCP VPC network. While the VPC itself is global, each subnet exists in a specific region and defines a CIDR range from which instances receive their internal IP addresses. Proper subnet design and IP planning are critical for performance, security, and avoiding costly rework later.

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What Is a Subnet?

A subnet (subnetwork) is a regional resource within a VPC. It defines a range of internal IP addresses using CIDR notation. Instances deployed in a particular region are placed into a subnet in that region.

Key properties:

• Regional — a subnet exists in one GCP region but is available across all zones within that region.
• CIDR range — each subnet has a primary IP range. You can also add secondary ranges for alias IPs (used by GKE pods and services).
• Expandable — you can increase (but not decrease) a subnet's CIDR range after creation without downtime.

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Primary and Secondary IP Ranges

Primary Range

Every subnet has a primary CIDR range. Instances receive their internal IP from this range. For example, a subnet with 10.0.1.0/24 provides 256 addresses.

Secondary Ranges