From Apple: Apple Ethernet Speed & Duplex Tool is an unsupported system extension that allows you to disable the auto negotiation and set the speed (10 Mbit or 100 Mbit) and duplex (Full or Half) for the built in Ethernet of an Apple iMac, a Power Macintosh G3 (Blue and White version only), or a 1999 PowerBook G3 (Bronze version only). This is intended for use on both server and client Macintoshes. This software is intended for Mac OS computers that are directly connected to a switch port and have disabled auto negotiation on that port. Do not use this software if this does not apply to you. If you are unsure, contact your network administrator. Apple Ethernet Speed & Duplex Tool (Duplexer) is an unsupported tool and therefore is not supported by Apple Computer, Inc.
read more +