Nintendo Network ID is a unique identifier made up of 6 to 16 characters that allows you to access Nintendo Network services, such as Miiverse and Nintendo eShop, on Wii U.
As of 10th December 2013, it is also possible to register a Nintendo Network ID on Nintendo 3DS.
If you have connected your Wii U to the Internet, we strongly recommend you register the same Nintendo Network ID on Nintendo 3DS. Using the same Nintendo Network ID on both systems will combine their Nintendo eShop balances.
For more information, please visit our Nintendo Network ID Support section.