Because my router is so new, dd-wrt does not offer the option to set speed/duplex in the VLANs menu. However, I've found that you can use the Network Bridge function on a Windows machine with multiple network interfaces. The properties of the virtual "bridge" device contain a menu for setting to 10/100/1000 and full/half duplex, among many more options. Thanks to this, I was able to connect my SE/30 to the internet through my Thinkpad. I still have no method to download files in MacWeb 2.0, currently.
