Routing complexity: Overlapping CIDR blocks for public and private subnets can create complex routing challenges within the VCN. Routers might get confused and misdirect traffic due to ambiguous address spaces.
Security risks: Overlapping ranges increase the potential for accidental exposure of private resources to the public internet. This can have significant security implications.
Best practices: Industry best practices and security guidelines always recommend clear separation between public and private address spaces within a VCN to ensure proper routing and maintain a strong security posture.
While the other options have their nuances, they are generally considered acceptable approaches:
A). Use the same CIDR block for both public and private subnets: This can be done, but it is not the most common or recommended approach. While technically possible, it can lead to routing complexity and make it harder to manage security policies separately for public and private networks.B. Allocate separate CIDR blocks from the same pool for public and private subnets: This is a common and recommended approach. It provides clear separation between public and private address spaces, simplifies routing, and facilitates better security management.D. Allocate CIDR blocks based on the expected number of instances in each subnet: This is considered a good practice for optimizing resource utilization. Allocating larger CIDR blocks than needed wastes address space, while smaller blocks might need subnet splitting later if the instance count grows.