A few more questions regarding RIS

I am not sure if this is the right forum or not, but if it is not could
someone please point me to the right forum.

I have a few questions regarding WDS/RIS

First of all is it mulicast or unicast by default? and are you able to
change it.

Also does the server have to authorized in DHCP in order to work? I removed
the authorization of the server in my virtual lab and it still works so I was
just wondering if it has to be authorized or if you can get by with out
authorizing it and what sort of problems may occur?

lastly, this will be used mostly in legacy mode as our organization has not
switched to vista yet so I have been using RIPrep to create the images, so
from the xp machine I just run:
\\Server_name\REMINST\Admin\I386\RIPrep.exe

Is there away to put permissions on that so only certain people can run that
command?

Thank you

Eric