With the wide adoption of wireless communication technologies, the current networking design of the Internet architecture has shown some limitations. Restricted by inherent layering constraints, valuable networking information cannot flow freely inside the network stack and potential operational optimizations are impossible to achieve. To overcome these limitations, we extend the current trend of cross-layer approaches with a framework called underlay protocol fusion: the basic building blocks of Internet functionality are factorized out and merged in a function pool where information sharing and operational optimizations are performed. To illustrate our approach, we present LUNARng (LUNAR next generation). It is a fully distributed underlay protocol designed for the Internet integration of wireless ad hoc networks (MANETs) where fundamental services such as name resolution, address autoconfiguration, and IPv4/IPv6 routing are transparently available whether the MANET is connected or not to the Internet. Internet integration refers here to the ability to insert/remove a MANET into/from the logical organization of the Internet without any loss of functionality. Moreover by using protocol models, the underlay nature of LUNARng allows to optimally merge (with respect to the multi-hop nature of MANETs) network operations which are traditionally carried out at different layers of the protocol stack.