halacha - Why is a utensil required when washing one's hands after bathroom use?

I am under the impression that one is required to wash one's hands with a utensil (typically, the same type of cup used to wash before bread) after using the bathroom.

• Firstly, what is the source of this custom?


• Secondly, why must a utensil be used when washing one's hands after using the bathroom?


• Lastly, is it permissible to wash one's hands without a utensil (after using the bathroom)?