Is there a programatic way to half a CIDR block in Terraform?

Question

I have a CIDR block, and I would like to exactly half it, so that I have two ranges that I can use within my Terraform project.

e.g. 10.10.10.0/24, which includes IPs in range 10.10.10.0 - 10.10.10.255 can be split to 10.10.10.0/25 & 10.10.10.128/25

I've tried looking at this with cidrsubnet function, but I don't really understand how to use it to get from the first range to the second range.

Any help on this would be appreciated!

Answer 1

This can be done with cidrsubnet as you pointed out.

output "first_half" {
  value = cidrsubnet("10.10.10.0/24", 1, 0) # 10.10.10.0/25
}

output "second_half" {
  value = cidrsubnet("10.10.10.0/24", 1, 1) # 10.10.10.128/25
}

Explenation:

cidrsubnet takes 3 arguments: cidrsubnet(prefix, newbits, netnum).

• The prefix is the actual CIDR range you want to cut in half.
• newbits is the number of additional bits which you want to extend the prefix. For example, if you have /24 prefix and you want to have /25 prefixes, newbits is the difference between 25 and 24: 25 - 24 = 1
• netnum is a whole number which is no more than newbits binary digits. In our case it can be either 0 or 1. 0 will be the range of 10.10.10.0/25, while 1 will represent the second half, being 10.10.10.128/25.

To give you another example, which makes it more understandable, lets cut 10.10.10.0/24 if 4 ranges:

We know, that we need /26 ranges, so 26 - 24 = 2 for the newbits. For netnum, we can have 0, 1, 2, 3, for which in the binary digits are 00, 01, 10, 11.

cidrsubnet("10.10.10.0/24", 2, 0) # 10.10.10.0/26
cidrsubnet("10.10.10.0/24", 2, 1) # 10.10.10.64/26
cidrsubnet("10.10.10.0/24", 2, 2) # 10.10.10.128/26
cidrsubnet("10.10.10.0/24", 2, 3) # 10.10.10.192/26