The jailbreak simply allows stuff to be run.
Then one installs usbnetworking from the right thread
In a nutshell: The USB on the PC end needs an IP assigning to it.
This must match what is in the usbnet config in [/mnt/us/]usbnet/etc/config
being on a different subnet (192.168.x.blah) to your main LAN doesn't matter.
however the subnet of the PC (USB) IP and the Kindle must match.
or 192.168.yaddayadda.1 192.168.yaddayadda.2
(192.168.15.201 and 192.168.15.1 would be a "real world" example)
If you use windows it can be a bumpain to do the initial config on the PC but once it is done you should be golden.
EDIT: Don't mistake your main LAN's IP address as your PC side IP address for the USBNET connection... It isn't - to reiterate... you need to assign an IP to your USB0 connection...
Here is another way:
(Hint: SSH works just as good over Wifi.)
connect to wifi on kindle.
go to 411? or 711? page (IP INFO PAGE) find ip... (Typically 192.168.1.low-number)
on PC: (for windows use putty) on linux ssh email@example.com
you will need to press enter when prompted for pw (will be blank)
that should be easiest. Both ways (usb or Wifi) should work just fine
(That is a mail I sent recently)
Hope that helps somewhat.