This can be as simple as the machine name where your preseed files reside up to a full URL. Any of these could be made to work:
  intra		[for example.com, these three are equivalent]
  intra.example.com
  http://intra.example.com/d-i/./lenny/preseed.cfg
  http://192.168.0.1/~phil/test47.txt
  floppy://preseed.cfg
  file:///hd-media/kiosk/./preseed.cfg
Ayagi yezmer ad yifsus am yisem n tmacint anida llan ifuyla-inek·inem n twila tuzwirt alamma d URL ummid. Yal yiwen seg wigi yezmer ad yeddu:
  intra		[d ameday.com, tlata-a kifkif-nsen]
  intra.example.com
  http://intra.example.com/d-i/./lenny/preseed.cfg
  http://192.168.0.1/~phil/test47.txt
  floppy://preseed.cfg
  file:///hd-media/kiosk/./preseed.cfg