$ vim subnet.tf
resource "google_compute_subnetwork" "subnetwork_external_dual" {
  name = "subnetwork-external-dual"

  ip_cidr_range = "10.0.4.0/22"
  region        = "us-central1"

  stack_type       = "IPV4_IPV6"
  ipv6_access_type = "EXTERNAL"

  network = google_compute_network.vpc_network_dual.self_link
}