I have a domain that needs to redirect from
I know this is possible, but how? It isn't automatic. What DNS records do I need to update?
Update your DNS with a CNAME record.
or in your case:
For more information, look at wikipedia's description
If you just want www.subdomain.domain.net to work then just setup DNS and setup the web server to respond to that host header as well as the normal one.
If you want www.subdomain.domain.net to actually change in the browsers URL bar to subdomain.domain.net that'll require the DNS change and the web server being configured to return 301 responses along with the new URL to redirect to.