Valid VPC CIDRs for an Existing VPC

Answer - A and D.

The AWS documentation mentions the following.

When you create a VPC, you must specify an IPv4 CIDR block for the VPC.

The allowed block size is between a /16 netmask (65,536 IP addresses) and /28 netmask (16 IP addresses)

For more information on VPC Sizing, please refer to the below link:

To answer this question, we need to understand what a VPC CIDR is and the rules for adding CIDRs to an existing VPC.

A VPC CIDR (Classless Inter-Domain Routing) is a range of IP addresses that defines the private IP address space in a VPC. The CIDR is specified in CIDR notation, which is a way of representing IP addresses and their associated network prefixes.

When creating a VPC, we define the CIDR block for the VPC. This CIDR block defines the range of IP addresses that can be used in the VPC. Once the VPC is created, we can add additional CIDRs to the VPC.

The rules for adding CIDRs to an existing VPC are as follows:

1. The CIDR block must not overlap with the existing CIDR blocks in the VPC.
2. The CIDR block must be within the private IP address ranges specified in RFC 1918.
3. The CIDR block must be between /28 and /16 in size.

Based on these rules, the valid VPC CIDRs to add to an existing VPC are:

B. 10.0.0.0/29 - This CIDR block is within the private IP address range specified in RFC 1918 and is between /28 and /16 in size. It also does not overlap with the existing CIDR block in the VPC.

D. 20.0.0.0/27 - This CIDR block is within the private IP address range specified in RFC 1918 and is between /28 and /16 in size. It also does not overlap with the existing CIDR block in the VPC.

A and C are not valid CIDRs to add to an existing VPC because:

A. 10.0.0.0/24 - This CIDR block overlaps with the existing CIDR block in the VPC (assuming the existing CIDR block is larger than /24).

C. 20.0.0.0/10 - This CIDR block is outside the private IP address ranges specified in RFC 1918.