Resource discovery in structured and unstructured peer-to-peer (P2P) systems have been extensively studied. Surprisingly, most of the systems are not designed to take advantage of node heterogeneity. In this paper, we propose a novel overlay called RATTAN, which serves as an underlay for unstructured P2P networks. RATTAN exploits the heterogeneity of nodes by structuring capable nodes as the core network of an unstructured P2P system. With RATTAN as the underlay, the scope of resource discovery in an unstructured P2P system can be maximal using a minimal number of messages. We evaluated RATTAN in simulation. The results show that (1) RATTAN is robust by exploiting redundant overlay links, and (2) the maximum bandwidth for protocol processing in a single RATTAN overlay is around 1 Mbits/sec, where 80% of nodes merely take 66 Bits/sec. We believe that a desktop machine equipped with an 100 Mbits/sec network interface is capable of processing 1 Mbits/sec of protocol operations. Peers that connect to the overlay via slow access networks, e.g. modems with 56 Kbits/sec, can accommodate the 66 bits/sec of overhead.