AWS Certified Advanced Networking Specialty Practice Exam

What is the correct CIDR format for a VPC subnet that needs to accommodate over 250 addresses?

1. 192.168.1.0/24

2. 192.168.1.0/25

3. 192.168.1.0/23

4. 192.168.1.0/22

The correct answer is: 192.168.1.0/23

To accommodate over 250 addresses in a VPC subnet, it's essential to choose a CIDR block that allows for sufficient IP addresses. When determining the number of usable addresses in a subnet, the formula to calculate usable IP addresses is 2^(32 - subnet mask bits) - 2. The "-2" accounts for the network and broadcast addresses, which cannot be assigned to hosts. For the CIDR block 192.168.1.0/23: - The subnet mask is 23 bits, meaning there are 32 - 23 = 9 bits left for host addresses. - Using the formula, this gives us 2^9 - 2 = 512 - 2 = 510 usable addresses. This means that a /23 subnet can accommodate significantly more than 250 IP addresses, making it suitable for the requirement outlined in the question. Other CIDR blocks provided do not meet the requirement: - A /24 provides 256 total addresses, which after accounting for the network and broadcast addresses leaves only 254 usable addresses—less than needed. - A /25 provides just 126 usable addresses, insufficient for the requirement. - A /22 yields 1022 usable addresses, which exceeds the need but also