One feature common to most existing routing protocols for wireless mobile ad hoc networks, or MANETs, is the need to flood control messages network-wide during the route acquisition and maintenance process. Flooding of control messages may result in redundant broadcasts and cause serious contention and collision problems in MANETs. In this paper, we develop an analytical model to study the performance of plain- and probabilistic flooding in terms of its reliability and reachability in delivering packets. Reliability is a measure of the total number of packets received by network nodes whereas reachability refers to the total number of unique nodes reached by the flooding process. We also provide simulation results to validate the model.