Uhh...I'm not quite sure what you mean, but it doesn't necessarily require you to buy anything else to get online. Depending on your router, that is. Thing is though, Mario Kart DS is questionable when it comes to the fun level of playing online. The VAST majority of matches end in one of two kinds of cheating, Snaking or Powering Off. Snaking is an illegal technique (though, some question whether it really is illegal) where people will do the drift all the way through the game, which makes them worm their way from side-to-side of the track, in a snake pattern (hence, snaking). Powering off is not debatable in terms of legality. You turn off your DS when you're about to lose, or are simply getting your ass whomped.
It's a good game to play with online friends. But it's really not too much fun if you're not playing with people you know.