In this paper, we present for mobile ad hoc networks an efficient distributed address allocation protocol which is immune to topology changes caused by node's mobility. Contrary to the common belief that mobility makes protocol design more difficult, we show that node's mobility can, in fact, be useful to provide efficient address allocation in ad hoc networks. In our protocol, each node that has been assigned an address manages a disjoint subset of free addresses independently. By taking advantage of node mobility, we can achieve roughly even distribution of free addresses amongst nodes in the system, which enables a new joining node to be configured by its neighbors via only local communication. Theoretical analysis and extensive simulation results are presented. We show that most of the address allocation requests can be processed in a timely fashion via local communication in the requester's neighborhood with time and message complexity in the order of node's degree, regardless of the network size.