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The standard IEEE 802.11 has been shown to be quite inefficient in multihop networks. In addition to the hidden-terminal and exposed-terminal problems, there is also an unreachability problem, which may result in link/routing failures and unfairness among multiple traffic flows. In this paper, a medium access control (MAC) protocol, called eMAC, is proposed. Under the proposed scheme, stations maintain double-hop neighborhood (DHN) graphs while exchanging designated e MAC tables to share their knowledge about their neighborhood topology. Using a DHN graph and an adaptive unreachability reporting mechanism, stations are reliably informed about their neighbors' unreachability status. Hence, they avoid establishing link-layer connections with their unreachable neighbors, and consequently, network resources are not consumed for unsuccessful connection-establishment efforts. Furthermore, we propose an adaptive table broadcasting technique to facilitate topology information dissemination in mobile ad hoc networks (MANETs). The performance of the proposed schemes is evaluated and compared with that of earlier schemes through simulations. Our results show a performance enhancement due to better handling of unreachability, possible heterogeneous power distributions among contending stations, and mobility issues.