Best approach for url rewrite / mask

I am currently working on a web project where the following problem has come up.

I have urls that i want to change like the following:

my starting url: http://ift.tt/2h14fv1

and i want to end up with this: http://ift.tt/2i5tJoz

but i want the browser to show the page from http://ift.tt/2h14fv1

So do i use rewriting or do i mask some parts of the urls. Also i cannot change my original url links. They need to stay as they are.

How do i do to achieve this?

Does anybody have a good tip for me?

Many thanks in advance.