In Brave New World, John is alienated in and exiled from both worlds. He has no home.
Remember, there's two kinds of civilizations in Brave New World, the technological and the primitive, both of which are dystopias.
Brave New World is a novel of ideas: it's not about plot or character. There's no realism in either. It's a novel of extremes. Like Shakespeare's tragic heroes, who cannot live with or without love, John is a romantic who cannot reconcile either extreme world. In other words, John loves them both. And hates them both. Through satire, Huxley is saying, therefore, he (and we) should avoid them both.
John is segregated from the Brave New World as a child. He grows up on the Savage Reservation. He hates it there. There's no law, no family, no education. His mother prostitutes herself, and John lives in shame. His problems there are just like the his problems will be in the Utopia, only without the technology.
John becomes a Byronic Hero is the Brave New World: he is the misunderstood problem-child of the primitive and the technological. At first, the Alphas are mesmerised by his Christ-like virtues. But then, after he is corrupted, they abandon him, just like his father did as a child. Technology is a red herring, according to Huxley. It doesn't solve problems: it only distracts us from them.
So, Huxley is saying that we should avoid both extremes, both dystopias. I think he wants his readers to live with a healthy balance of technology, pleasure, sports, and pharmaceuticals--if that can be done. Regardless, we cannot exile ourselves, like John, in either world because it only leads to tragedy. We need family, monogamous relationships, education, and religion to ground us.
I agree with what the first answer says, but I don't think it answers the question -- or at least not how I understand this question. I think you are asking whether the savages are on the reservation to protect them from civilization or to protect civilization from them.
I think that the civilization is being protected from the savages. To me, this is the only way it makes sense because contact with the savages might change individual civilized people and those changes might spread.
I do not think that the opposite change is something you need to worry about. If the savages see civilization and want it for themselves, that's not going to endanger them. It's not like they can go back to their village and change it to be like civilization. The civilization depends so much on technology that the savages could never start having civilization. By contrast, the savages' culture depends only on ideas and civilized people could have those.