The Wii Number is the 16-digit friend code of your Wii, which can be registered with other Wiis to exchange mail via WiiConnect24 (and now RiiConnect24).

It's made up of a few variables that identify your Wii:

  • Hollywood ID (chip in the Wii)
  • Counter (increments every time you format your Wii, can range from 0-31)
  • Hardware Model (RVL, RVT for dev hardware...)
  • Area Code (Region - USA, EUR, JPN...)
  • Checksum

These variables are then scrambled using a ton of complicated bitwise operations.

RiiConnect24 has source code to encode and decode a Wii Number. It has been ported to C, Golang, and Python.