Convert your Mac to modern RJ45 ethernet!
This adapter/transceiver plugs into the AAUI ethernet port on your Mac. A standard RJ45 ethernet cable plugs into the other end to convert or adapt your Mac to RJ-45 ethernet. The AAUI ethernet port was introduced with the Quadra 700 and 900, it was a standard port on most Macs until discontinued with the G3. Some PowerBooks and LaserWriters also have this port.
I have several flavors of these transceivers - Apple Ethernet Twisted Pair Transceiver M0437 (pictured), Asante FriendlyNet 10BASET, Farallon Ethernet Transceiver, and DaynaPort TRX-T. I have only 1 of each except the Asante. If you have a preference and it's still in stock, let me know.