code: func determineSubnetURL(service *compute.Service, networkProjectID, networkName, region string) (string, error) {
  	subnets, err := listSubnetworksOfNetwork(service, networkProjectID, networkName, region)
  	if err != nil {
  		return "", err
  	}
  
  	autoSubnets, err := subnetsInCIDR(subnets, autoSubnetIPRange)
  	if err != nil {
  		return "", err
  	}
  
  	if len(autoSubnets) == 0 {
  		return "", fmt.Errorf("no subnet exists in auto CIDR")
  	}
  
  	if len(autoSubnets) > 1 {
  		return "", fmt.Errorf("multiple subnetworks in the same region exist in auto CIDR")
  	}
  
  	return autoSubnets[0].SelfLink, nil
  }
docs: // determineSubnetURL queries for all subnetworks in a region for a given network and returns
  // the URL of the subnetwork which exists in the auto-subnet range.