1. A process for generating a “child/parent” hierarchical tree to carry out a point-to-multipoint routing in a Mobile Ad Hoc Network (MANET), wherein the process consists:, each time that a “child” router (RR, R) of said network wishes to join the hierarchical tree associated with a source router (SR), said child router sends a join message that associates said child router with a neighboring “parent” router (R) of said child router in the network, wherein said child router uses its knowledge of a network topology to determine said parent router as a neighboring router better placed than said child router on a routing path to the source router, said join message being transmitted by said child router to said neighboring parent router (R) in point-to-point mode, said join message denoting said source router (SR), and (a) If said neighboring parent router is already associated with another child router of said network, said requesting child router is associated with said neighboring parent router, in order to join it to said tree, and a value of a state information representative of a number of child routers associated with said neighboring parent router is incremented by one unit, (b) If said neighboring parent router is not already associated with another child router of said network, said requesting child router is associated with said neighboring parent router and said state information of said neighboring parent router is given the value “one”, and said neighboring parent router transmits a join message designating said source router (SR) in point-to-point mode, to another neighboring router in said network, which in its turn constitutes a parent for said neighboring parent router, (c) Operation a) or operation b) is then repeated with said neighboring parent router receiving a join message according to whether or not it is already associated with another child router of said network.