code: func (c *ComputeUtil) ip() (string, error) {
  	instance, err := c.service.Instances.Get(c.project, c.zone, c.instanceName).Do()
  	if err != nil {
  		return "", unwrapGoogleError(err)
  	}
  
  	nic := instance.NetworkInterfaces[0]
  	if c.useInternalIP {
  		return nic.NetworkIP, nil
  	}
  	return nic.AccessConfigs[0].NatIP, nil
  }
docs: // ip retrieves and returns the external IP address of the instance.