Terraform cidrsubnet function troubleshoot

Question

I have a situation where I am trying to create some subnets dynamically with the cidrsubnet function in Terraform. Here is how I have it defined:

data "aws_availability_zones" "availability_zones" {
  exclude_names = [
    "us-west-2c",
    "us-west-2d"
  ]
}
locals {
  availability_zone_names = "${data.aws_availability_zones.availability_zones.names}"
}

resource "aws_subnet" "public_subnets" {
  count = "${length(local.availability_zone_names)}"
  vpc_id = "${aws_vpc.vpc.id}"
  cidr_block = "${cidrsubnet(aws_vpc.vpc.cidr_block, 11, count.index)}"
  availability_zone = "${local.availability_zone_names[count.index]}"
  map_public_ip_on_launch = true

  tags {
    Name = "${var.name}-public.${local.availability_zone_names[count.index]}"
  }
}

I am only trying to create these subnets in two availability zones in the region (hence why I have 2 AZs being excluded. Upon applying this code, I am getting the result I want in one of the subnets I am trying to create. However the cidr block address for the other subnet I am trying to get an address of 10.32.2.0/27 but the cidrsubnet function is generating 10.32.0.32/27. Any advice where I went wrong would be helpful.

Planned Output

aws_subnet.public_subnets[1]
  id:                              <computed>
  arn:                             <computed>
  assign_ipv6_address_on_creation: "false"
  availability_zone:               "us-west-2b"
  availability_zone_id:            <computed>
  cidr_block:                      "10.32.0.32/27"

Marcin Marcin · Accepted Answer · 2020-11-16T23:40:44

If you want to use prefix /27 and want your subnets to be:

10.32.0.0/27
10.32.2.0/27
10.32.4.0/27

and so on, then you have to multiply your index by 16, as these CIDR ranges are not consecutive:

cidr_block = "${cidrsubnet(aws_vpc.vpc.cidr_block, 11, count.index * 16)}"

Terraform cidrsubnet function troubleshoot

1 Answers