more on apt -- what preferences?

[andrej hocevar <ah@siol.net>]

Hello.
Lately I've added some cds (woody from linuxiso.org) to my
sources.list. I'm running woody installed from the net; the ftp
sources are also in that file. Since quite a few packages I wanted
to install were grabbed off the net, I assume that those weren't on
my cds -- following some previous mails, we can't tell why there
should be 8 in total, and I only have three. However, if a package
is on them, will it be installed from that very source or will apt
almost always switch to ftp? Merely because of a newer version? Can
one set the preferences for that? (I installed woody at a friend's, 
no use in doing it over slow ppp, that's the reason I would
primarily like to use cds.) 

andrej