there is more information here:
http://archicad-talk.graphisoft.com/viewtopic.php?p=5198ᑎ
which may or may not help you. the other thing you can do is reset your plotter to factory defaults. then in the manual somewhere it should tell you the IP address that it ships with as default.
you can then go in with the terminal (see the link above) and change it. remember that to do this you need a computer on the same sub-net, so you may have to temporarily configure one of your machines to do this . . .
HTH
~/archiben
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