The narrator of the novella is most definitely an all-knowing narrator. He's not restricted to one character's point of view, but can get into the mind of any character. When he does, in fact, he even reveals himself to be a first-rate psychiatrist (see "Tone"), and seems to know what's really going on in the recesses of the characters' psyches.
By choice, the narrator often limits himself to Ivan's perspective and presents other characters from Ivan's point of view. This makes sense given that the story is about Ivan, after all. But whenever it's helpful, our narrator will jump out of Ivan's head and tell us what's going on in someone else's. Case in point: Ivan's death at the very end of the book, when the narrator is able to tell us both what Ivan himself is experiencing and what everyone else sees him going through.