UPD: take vpc cidr from variable and calculate subnets

vpc-subnets/var-def.tf

@@ -10,3 +10,6 @@ provider "aws" {
 }
 
 variable "default-tags" {}
+variable "vpc-cidr" {
+  default = "172.16.0.0/16"
+}

vpc-subnets/vpc.tf

@@ -1,5 +1,9 @@
 data "aws_availability_zones" "available" {}
 
+locals {
+  subnet_start = cidrsubnets(var.vpc-cidr,4,4) 
+}
+
 module "random" {
   source = "./m.random"
 }
@@ -9,10 +13,10 @@ module "vpc" {
   version = "2.6.0"
 
   name                 = "demo-vpc-${module.random.number}"
-  cidr                 = "172.16.0.0/16"
+  cidr                 = var.vpc-cidr
   azs                  = data.aws_availability_zones.available.names
-  private_subnets 	= cidrsubnets("172.16.18.0/23", 1, 1)
-  public_subnets 	= cidrsubnets("172.16.20.0/23", 1, 1)
+  private_subnets 	= cidrsubnets(local.subnet_start[0], 4,4)
+  public_subnets 	= cidrsubnets(local.subnet_start[1], 4,4)
   enable_nat_gateway   = false
   single_nat_gateway   = true
   enable_dns_hostnames = true