code: public static SubnetworkId of(String region, String subnetwork) {
      return new SubnetworkId(null, region, subnetwork);
    }
docs: Returns a subnetwork identity given the region and subnetwork names. The subnetwork name must
  be 1-63 characters long and comply with RFC1035. Specifically, the name must match the regular
  expression {@code [a-z]([-a-z0-9]*[a-z0-9])?} which means the first character must be a
  lowercase letter, and all following characters must be a dash, lowercase letter, or digit,
  except the last character, which cannot be a dash.
  
  @see <a href="https://www.ietf.org/rfc/rfc1035.txt">RFC1035</a>