Current peer-to-peer (P2P) index structures only support a subset of the desired functionality for P2P database systems. For instance, some P2P index structures support equality queries but not range queries, while others support range queries, but do not support multiple data items per peer or provide guaranteed search performance. In this paper, we devise a novel index structure called P-Ring that supports both equality and range queries, is fault-tolerant, provides guaranteed search performance, and efficiently supports large sets of data items per peer. We are not aware of any other existing index structure that supports all of the above functionality in a dynamic P2P environment. In a thorough experimental study we evaluate the performance of P-Ring and quantify the performance trade-offs of the different system components. We also compare P-Ring with two other P2P index structures, Skip Graphs and Chord.