There actually is a 18-Speed IG drive that is on the market: the pinion drive.

Only a few framebuilders incorporate this in their frames, but they do exist...

Why not have the gears inside the bottom bracket?
I know that a PC can have multiple network adapters each of which with different network identifiers (IP,MAC etc.). 

My question is, can a certain PC have more than one active default gateway? And if so, how does the PC (or better say the OS) knows to what adapter use for any given packet?
