Terraform Validate CIDR. Checks that the format is correct, and that the prefix matches the first IP in the range (thus it's a valid prefix)

demo.tf

variable "demoIpv4Cidr" {
  type    = string
  default = "10.0.0.1/16" # try "10.0.0.0/16" for a valid value or "10.0.0.x/16" for an invalidly formatted cidr

  validation {
    condition     = (
      can(cidrhost(var.demoIpv4Cidr, 0)) &&
      try(cidrhost(var.demoIpv4Cidr, 0), null) == split("/", var.demoIpv4Cidr)[0]
    )
    # the above could be simplified to:
    # condition = cidrhost(var.demoIpv4Cidr, 0) == split("/", var.demoIpv4Cidr)[0]
    # ...though that would throw `Error in function call` instead of `Invalid value for variable` 
    #    if an invalid cidr (10.0.0.x/16) were used
  
    error_message = "Must be valid IPv4 CIDR."
  }
}

output demo {
	value = try(cidrhost(var.demoIpv4Cidr, 0), null) 
}