As wireless devices become more pervasive, mobile ad hoc networks are becoming increasingly important, motivating the development of highly scalable ad hoc networking techniques. In this paper, we present the design and evaluation of a novel protocol for scalable routing in ad hoc networks, as part of the Safari project. Safari leverages and integrates research in both ad hoc networking and peer-to-peer networking. We develop a probabilistic, elf-organizing network hierarchy formation protocol that recursively forms the nodes of the ad hoc network into an adaptive, proximity-based hierarchy of cells. We develop a hybrid routing protocol that uses this hierarchy, with reactive and proactive routing, to scale to large number of nodes. The mapping of unique node identifiers to hierarchical addresses is done using a distributed hash table that leverages the hierarchical network structure. We evaluate this design through analysis and simulations, under increasing network size, increasing fraction of mobile nodes, and increasing offered traffic load. Our analysis is well matched by our simulations, and our results demonstrate the protocol's scalability.