Uthernet On a IIgs
After installing in Slot 1 with the chip side of the board facing away from the power supply, I went into the IIgs control panel (control-openapple-escape) and made sure Slot 1 was set to "Your Card", and continued with the remote booting process described here. Contiki 1.3 obtained from the a2RetroSystems site, runs fine right out of an AppleShare mount. Marinetti and the Uthernet driver were also obtained from the a2RetroSystems site and installed onto the remote booted AppleShare without trouble.
Contiki needs the interface to be configured. I went to the Directory icon on the Contiki desktop, and selected the configuration editor. Within the configuration editor, set the IP, netmask, gateway, DNS, and it's important not to forget setting the slot number the Uthernet got installed in. Once setup, Contiki offers a web browser, email, telnet, and irc clients as well.
The Uthernet card would not work for me with a stock 1MB Applied Engineering GSRAM card installed in the memory slot. However, a Briel Computers 4MB GSRAM card worked great.