How many IP addresses does OCI reserve in each subnet?

OCI reserves the first two addresses (network address and a reserved address) and the last address (broadcast) in each subnet, totaling three reserved addresses. For example, a /24 subnet has 253 usable addresses out of 256. This is fewer reserved addresses than AWS (which reserves five), giving you slightly more usable IPs per subnet in OCI.

Should I use regional subnets or AD-specific subnets?

Oracle recommends regional subnets for most workloads. Regional subnets span all availability domains in a region, simplifying management and providing more flexibility for resource placement. AD-specific subnets are scoped to a single availability domain and are primarily needed for legacy configurations. Regional subnets also make it easier to use regional load balancers and simplify CIDR planning since you need fewer subnets.

Can I add more CIDR blocks to an existing VCN?

Yes, OCI supports up to five CIDR blocks per VCN, and you can add or remove secondary CIDR blocks after VCN creation without downtime. The new CIDR block cannot overlap with existing VCN CIDRs or with CIDRs of peered VCNs. This feature is useful for extending address space when your original CIDR runs out of room for new subnets.

OCI VCN CIDR Planner

NetworkingOCI

Plan VCN and subnet CIDR allocation with availability domain awareness.

VCN Subnet Configuration

VCN CIDR Block

Number of Subnets

Subnet Prefix Length

Subnet Type

OCI reserves 3 IPs per subnet: network address, default gateway, and broadcast address. VCN CIDR must be between /16 and /30.

Raw Output

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About This Tool

Planning CIDR blocks for OCI Virtual Cloud Networks requires careful consideration of subnet sizes, route aggregation, peering constraints, and future growth. OCI VCNs support multiple CIDR blocks (up to five per VCN) and subnets can be regional or availability-domain-specific. The VCN CIDR Planner helps you design IP address schemes that avoid overlapping ranges, accommodate required subnet sizes for each tier, and leave room for expansion. It calculates usable addresses per subnet (accounting for OCI's reserved addresses) and validates that your plan supports VCN peering and on-premises connectivity.

When to Use This Tool

•Designing a multi-tier VCN layout with separate subnets for web, application, and database tiers across three availability domains
•Planning non-overlapping CIDR blocks for multiple VCNs that will be connected via Local Peering Gateways or a DRG
•Calculating subnet sizes that accommodate expected instance counts plus headroom for autoscaling and future growth
•Validating that planned VCN CIDRs do not conflict with on-premises address ranges connected via FastConnect or IPSec VPN

Frequently Asked Questions

How many IP addresses does OCI reserve in each subnet?: OCI reserves the first two addresses (network address and a reserved address) and the last address (broadcast) in each subnet, totaling three reserved addresses. For example, a /24 subnet has 253 usable addresses out of 256. This is fewer reserved addresses than AWS (which reserves five), giving you slightly more usable IPs per subnet in OCI.
Should I use regional subnets or AD-specific subnets?: Oracle recommends regional subnets for most workloads. Regional subnets span all availability domains in a region, simplifying management and providing more flexibility for resource placement. AD-specific subnets are scoped to a single availability domain and are primarily needed for legacy configurations. Regional subnets also make it easier to use regional load balancers and simplify CIDR planning since you need fewer subnets.
Can I add more CIDR blocks to an existing VCN?: Yes, OCI supports up to five CIDR blocks per VCN, and you can add or remove secondary CIDR blocks after VCN creation without downtime. The new CIDR block cannot overlap with existing VCN CIDRs or with CIDRs of peered VCNs. This feature is useful for extending address space when your original CIDR runs out of room for new subnets.

Related Learning Guides

OCI VCN Networking Deep Dive25 min read

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