code: func DefaultServiceIPRange(passedServiceClusterIPRange net.IPNet) (net.IPNet, net.IP, error) {
  	serviceClusterIPRange := passedServiceClusterIPRange
  	if passedServiceClusterIPRange.IP == nil {
  		klog.Infof("Network range for service cluster IPs is unspecified. Defaulting to %v.", kubeoptions.DefaultServiceIPCIDR)
  		serviceClusterIPRange = kubeoptions.DefaultServiceIPCIDR
  	}
  	if size := ipallocator.RangeSize(&serviceClusterIPRange); size < 8 {
  		return net.IPNet{}, net.IP{}, fmt.Errorf("The service cluster IP range must be at least %d IP addresses", 8)
  	}
  
  	// Select the first valid IP from ServiceClusterIPRange to use as the GenericAPIServer service IP.
  	apiServerServiceIP, err := ipallocator.GetIndexedIP(&serviceClusterIPRange, 1)
  	if err != nil {
  		return net.IPNet{}, net.IP{}, err
  	}
  	klog.V(4).Infof("Setting service IP to %q (read-write).", apiServerServiceIP)
  
  	return serviceClusterIPRange, apiServerServiceIP, nil
  }
docs: // DefaultServiceIPRange takes a the serviceIPRange flag and returns the defaulted service ip range (if  needed),
  // api server service IP, and an error