Generating the same random number from a string across clients

I'm building a multiplayer desktop Air game, and I need to generate a random number that is the same in both clients.

I've been reading a bit about how other games do it, and apparently if I have the same seed in both clients, the random output will be the same.

However I never tried to do this, no idea where to begin.

How does it work?, how does this string (let's same player name) assures me that the random output in both clients will be the same?.