Always redirect non www urls, rather than alias them.

I've just noticed that our site is showing up on the search engines as two listings, one for the url: http://ourdomain.com.au and a separate one for http://www.ourdomain.com.au This is interesting becasue our site operates on a remotely hosted Microsoft IIS server, and when I asked them to make the non www version work, I noticed that the address in the browser didn't change when I entered it in without the www prefix. I thought "no worries" and preceeded to ensure that all links on the front page specified the full path with the www prefix. I also try to make all external links into the site include the www prefix aswell, and there seemed to be no problem, but now I've noticed this other listing for our site, and it's ranking higher than us, but is not indexed. I've just asked the web server guys to put the IIS equivalent of a 301 redirect in instead. I've noticed that this is what Google and many other large sites seem to do.