If you were one of those people that had problems with the multiplayer then perhaps this will help you. I found out that these ports can/should be portforwarded when you play Nintendo games on your internet and it fixed the multiplayer connection issues i've been having with Mario Kart 8.
TCP: 80, 443, 6667, 12400, 28910, 29900, 29901, 29920 UDP: (Range) 1-65535
Hopefully it will help others that may have problems!