Depending on your System version, one of the potentially most troublesome CPs/extensions is the invisible Network CP, a predecessor of the very visible AppleTalk CP. In System 7.5 and above the suspects are all clear: Sharing or Sharing Setup, AppleTalk, Users and Groups, TCP/IP and FileSharing. Network CP can variously play up, refusing to be replaced, shifted, or to give up and go away. Its function is selection between Classic Networking and AppleTalk/Ethernet, ie between serial and ethernet sharing, just as AppleTalk CP does. As you already know, whenever a (later) System has a difficulty with ethernet it defaults to Printer Port (serial) in the AppleTalk CP.

The various possibilities with System version are given by Matthew Glidden in, eg About This Particular Mac. He also has a site about networking.

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