Nintendo Network is an online service for the Nintendo 3DS and the Wii U, serving as a replacement for Nintendo Wi-Fi Connection, supporting Mario Kart 7 and Mario Kart 8 and temporally Mario Kart 8 Deluxe.
In Mario Kart 7, the Nintendo Network was used for online races and battles. It only can hold up to eight players per-room. Also, another change is that players now can have more than 9999 VR/BR(ex. 60257 VR). Also, players can view their friends and other players they played against.
Nintendo Network is used once again, in Mario Kart 8, now having 12 players per-room. Also, on the worldwide and regional modes, player can create rooms with custom rules. Players can also create tournaments for other players to join. Leaderboards from Mario Kart Wii also return. Along with posting Miiverse posts after a player beats a staff ghost, wins a Grand Prix, or when ever they want, they can also use Stamps in their posts.
Miiverse has been removed from the game, but still supports Nintendo Network. Stamps have also been removed. Nintendo Network is going to be replaced by Nintendo's paid online service, Nintendo Switch Online, in 2018. However it will be kept on the 3DS and Wii U.