Recently, the research on mobile ad hoc networks is departing from the view of stand-alone networks, to focus on hybrid self-organized network environments interconnected to the Internet. This type of networks is built on a mix of fixed and mobile nodes using both wired and multi-hop wireless technologies, and may be easily integrated into classical wired/wireless networking infrastructures. In this paper we design a lightweight and efficient architecture to build such a multi-hop hybrid ad hoc network, which will be used as a flexible and low-cost extension of traditional wired LANs. Our proposed architecture provides transparent global Internet connectivity and self-configuration to mobile nodes, without requiring configuration changes in the preexisting wired LAN. Differently from most of the implemented solutions, which are based on complex IP-based mechanisms, such as Mobile IP, IP-in-IP encapsulation and IP tunneling, our proposed system operates below the IP level, and employs only layer-2 mechanisms. We have prototyped the core functionalities of our architecture, and we present several experimental results to verify the network performance constraints, and how different OLSR parameter settings impact on them.