When restoring from an image via a network while started from a Mac OS X 10.3 disc, use the host computer's (or server's) numeric network addresses instead of a text address. For example, 125.32.19.105.
You can find the IP address in Network preferences on the computer that's hosting the image file(s), if it's a Mac OS X-based computer.
Don't use a host name or Bonjour (formerly "Rendezvous") name. If you do, Disk Utility may display this: "(51) an error occurred while copying".
Additional information
If a numeric network address isn't used and the "(51)" alert appears in Disk Utility, the install log also reports:
root: bootstrap_look_up(): unknown error code
root: asr couldn't mount source image.