code: def _create_subnet(self, resource_group_name, subnet_id, vnet_name):
          
          subnet_config = {'address_prefix': '10.0.0.0/29'}
  
          try:
              subnet_setup = self.network.subnets.create_or_update(
                  resource_group_name, vnet_name, subnet_id, subnet_config
              )
          except Exception as error:
              raise AzureCloudException(
                  'Unable to create subnet: {0}.'.format(error)
              )
  
          return subnet_setup.result()
docs: Create a subnet in the provided vnet and resource group.