What is a Route Table?

• A route table is a set of rules, called routes, that determines where network traffic is directed within a VPC (Virtual Private Cloud).
• Each VPC has a main route table by default, and you can create additional route tables for specific use cases.

Key Concepts:

1. Routes: Each route specifies a destination and a target. The destination can be an IP address range (CIDR block), and the target can be an Internet Gateway, NAT Gateway, VPC peering connection, VPN connection, etc.
2. Main Route Table: When you create a VPC, AWS automatically creates a main route table. All subnets that do not have an explicit route table associated with them will use this main route table.
3. Subnet Association: You can associate specific route tables with individual subnets. This allows for more granular control over the routing of traffic within the VPC.

Important Points:

1. Default Routes: By default, the main route table contains a local route that allows communication between instances in the VPC and may also contain routes to an Internet Gateway for public subnets.
2. Local Route: Every route table includes a local route (e.g., 10.0.0.0/16 -> local), which enables communication between instances in the VPC.
3. Multiple Route Tables: You can create multiple route tables within a VPC to route traffic differently for various subnets, allowing for more complex networking setups.
4. Route Propagation: If you use VPN or Direct Connect, you can enable route propagation, allowing the routes from the virtual private gateway to automatically populate the route table.

Best Practices:

1. Create Specific Route Tables: Use separate route tables for public and private subnets to manage their routes distinctly. Public subnets will need a route to the Internet Gateway, while private subnets might route through a NAT Gateway.
2. Use Descriptive Names: When creating route tables, give them descriptive names to indicate their purpose (e.g., "PublicSubnetRouteTable", "PrivateSubnetRouteTable").
3. Review Routes Regularly: Periodically review your route tables to ensure that they meet your current networking requirements and to remove any unnecessary routes.
4. Testing and Documentation: Document your route tables and the associated routes for future reference and troubleshooting. Test connectivity after changes to ensure that routes are working as expected.