Nintendo explained how they will be handling Mario Kart 8 DLC when it comes to online multiplayer.
An earlier report claimed that people who did not buy the DLC packs will not be able to play the DLC courses, triggering fears that players can get kicked out because they don’t have the DLC. However, the system has been set up to keep that from happening.
When the 1st batch of DLC arrives, you will be allowed to pick to play the original 32 courses, or go with all 40 courses. When running a tournament, you can pick to use the 32 original courses, all 40 courses, even just the DLC courses. When you pick to race with the DLC courses in any of these situations, you will only be able to race with other players who have the DLC courses.
When it comes to DLC characters, vehicles, and gear, however, everything has been more loosened up. You will be able to use any of these DLC items in online races in such a way that other players who did not buy the content will still be able to play with you and see you using and wearing them.
When DLC becomes a factor in online games, fragmentation becomes an issue, but Nintendo has taken care to minimize any ill effects incorporating DLC can have on the Mario Kart 8 playing experience.