When the host-guest adapter is marked as active in the networkport configuration then other Macs cannot find my shared printer. If I remove the active mark then they can print without any problems. Also XP has no problems accessing the Internet. So is this adapter unnecessary?
macgebruiker, Parallels Host-Guest adapter is only necessary if you run a VM in Host-Only mode. You can safely disable it in System Preferences until you really need to use Host-Only.
