I haven't done this, but I have looked into it a bit (I'd love to hear someone who has done this with Mandrake 9.0 or 9.1)...basically you make a boot disk with the network.img instead of the regular one.
Then what happens is that the disk will prompt you for a network location to install from (this is likely an ftp address...).
One thing that annoyed me when I tried the boot disk is that it doesn't give you a list of choices, its just a blank field...so you have to KNOW where you're going to install from over the net.
At the time this screwed me up since I had no PC up and running (which is what I was trying to install) so I couldn't just boot up and go find a good address. So I gave up and installed a RedHat CD I had lying around...(This was a couple of years ago tho...so the install may be a lot better now)
Still...if you can, get the CDs, they are worth having.