code: public Operation createSubnetwork(
        SubnetworkId subnetworkId, String ipRange, OperationOption... options) {
      return compute.create(SubnetworkInfo.of(subnetworkId, getNetworkId(), ipRange), options);
    }
docs: Creates a subnetwork for this network given its identity and the range of IPv4 addresses in
  CIDR format. Subnetwork creation is only supported for networks in "custom subnet mode" (i.e.
  {@link #getConfiguration()} returns a {@link SubnetNetworkConfiguration}) with automatic
  creation of subnetworks disabled (i.e. {@link
  SubnetNetworkConfiguration#autoCreateSubnetworks()} returns {@code false}).
  
  @return an operation object if creation request was successfully sent
  @throws ComputeException upon failure
  @see <a href="https://wikipedia.org/wiki/Classless_Inter-Domain_Routing">CIDR</a>