azure - When I run cidrsubnet function in terraform I am getting 'prefix extension of 3 does not accommodate a subnet numbered 8'. Why is this?

Question

code:

cidrsubnet("10.1.2.0/21", 3, 8)

question from:https://stackoverflow.com/questions/65838425/when-i-run-cidrsubnet-function-in-terraform-i-am-getting-prefix-extension-of-3

Answer 1

When you are using cidrsubnet("10.1.2.0/21", 3, 8), you are adding 3 bits. Since in binary 2 ^ 3 = 8, you can define maximally 8 subnets in this range: 0,1,2,..., 7 with the following cidrs:

• 10.1.0.0/24
• 10.1.1.0/24
• 10.1.2.0/24
• 10.1.3.0/24
• 10.1.4.0/24
• 10.1.5.0/24
• 10.1.6.0/24
• 10.1.7.0/24

If you want to define subnets between 0 and 15, you have to use: cidrsubnet("10.1.2.0/21", 4, 8), since 2 ^ 4 = 16 and you can have sixteen subnets: 0, 1, 2, ..., 15.