Route Table in OCI

A Route Table is a collection of Route Rules that tells OCI where network traffic should be sent when it leaves a subnet.

Example

If a private subnet needs internet access through a NAT Gateway, the subnet is associated with a Route Table that contains a route rule pointing to the NAT Gateway.

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Route Rule in OCI

A Route Rule is an individual entry within a Route Table. It defines:

• Destination CIDR Block (where the traffic is going)
• Target (where the traffic should be forwarded)

Example

Destination CIDR	Target
0.0.0.0/0	Internet Gateway

This route rule means that all traffic destined for the internet is sent to the Internet Gateway.

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Quick Example

Suppose a public subnet contains a web server:

Component	Value
Subnet	Public Subnet
Destination CIDR	0.0.0.0/0
Target	Internet Gateway

Traffic Flow:

Instance → Route Table → Route Rule (0.0.0.0/0) → Internet Gateway → Internet

Key Difference

Route Table	Route Rule
A collection of routing entries.	A single routing entry inside a Route Table.
Associated with a subnet.	Defines destination and target.
Can contain multiple route rules.	Represents one routing path.

One-line summary: A Route Table is a container of routing instructions, while a Route Rule is a single instruction that tells OCI where to send specific network traffic.

Steps to create custom Route Table

1. Navigate to Networking → Virtual Cloud Networks, select Hub_Network, click the Routing option, and then click Create Route Table.

2. Enter a name for the route table, select the appropriate compartment, and then click Create.

Scroll down and also select tag and finally click on Create

3. Navigate to the newly created route table, click Add Route Rules, and configure the route rule as required.

4. Select the Target Type, enter the Destination CIDR Block, choose the appropriate Compartment, and select the target Internet Gateway. Then click Add Route Rules.

5. The route rule has been successfully added to the custom route table, and the custom route table is now configured to route traffic according to the specified destination and target.

6. Now, navigate to Subnets, select the Public Subnet, and click Edit. Associate the subnet with the newly created custom route table instead of default route table, and then save the changes.

7. We can see that the Public Subnet is now associated with the custom route table that we have created instead of the default route table.

Explanation:-

When a subnet is created, it is typically associated with the VCN's default route table. By updating the subnet to use a custom route table, the subnet will follow the routing rules defined in the custom route table rather than the default one. This allows you to control how traffic is routed, such as directing internet-bound traffic through a specific Internet Gateway, NAT Gateway, or other network target.

Traffic Flow Before:

Public Subnet → Default Route Table → Route Rules in Default Route Table

Traffic Flow After:

Public Subnet → Custom Route Table → Route Rules in Custom Route Table

This ensures that the Public Subnet uses the routing configuration you explicitly defined in the custom route table.

Default Route Table vs Custom Route Table (OCI)

Feature	Default Route Table	Custom Route Table
Creation	Automatically created with the VCN	Manually created by the administrator
Management	Managed as the default routing option	Fully customizable based on requirements
Association	Used by subnets unless another route table is assigned	Must be explicitly associated with a subnet
Flexibility	Limited to common routing needs	Supports different routing policies for different subnets
Use Case	Simple network environments	Complex or segmented network architectures

Why Use a Custom Route Table if a Default Route Table Already Exists?

A custom route table allows different subnets to have different routing behaviors within the same VCN. This provides greater control, security, and network segmentation.

Example

• Public Subnet: Route traffic to an Internet Gateway for internet access.
• Private Subnet: Route traffic to a NAT Gateway without exposing resources directly to the internet.
• Hub-and-Spoke Network: Different subnets can route traffic through different gateways or DRGs based on business requirements.

Key Benefit

Custom route tables provide granular routing control, while the default route table is suitable only for basic routing requirements.