code: func StringToIPNetHookFunc() DecodeHookFunc {
  	return func(
  		f reflect.Type,
  		t reflect.Type,
  		data interface{}) (interface{}, error) {
  		if f.Kind() != reflect.String {
  			return data, nil
  		}
  		if t != reflect.TypeOf(net.IPNet{}) {
  			return data, nil
  		}
  
  		// Convert it by parsing
  		_, net, err := net.ParseCIDR(data.(string))
  		return net, err
  	}
  }
docs: // StringToIPNetHookFunc returns a DecodeHookFunc that converts
  // strings to net.IPNet