Post Under Post

How can i create a post under another post? is there a plugin for this?

example:

http://www.mydomain.com/my first post

and i want to create another post under it.

http://www.mydomain.com/my first post/2
http://www.mydomain.com/my first post/3

is this possible? if yes please help me how to do it or give me a link for this tutorial.