Sharding, or a second key, or detecting the iPad and using a lower number with an offset internally? Still requires special handling at one end or another.

NAT is a colossal pain in the ass. I shudder to think of the number of man-years that have been spent on NAT traversal. NAT breaks one of the fundamental functions of an IP address - a unique identifier for a network device. It turns a simple identifier into a weird, amorphous abstraction.

IPv6 isn't perfect, but we could have avoided a lot of hassle if we'd started off with it.

Sure. Now ping my computer at 10.3.1.4