Overlay networks are increasingly practical on the Internet. Its main applications are Peer-to-Peer (P2P) file sharing and P2P file streaming applications. They may generate undesirable traffic for Internet Service Providers (ISPs) and charge network load to specific physical links because of a mismatch between overlay networks and the underlay network. Overlay networks try to infer the properties of the underlay network from end nodes to enhance their performance. However, it is difficult to know the underlay network from end nodes. In this paper, we propose an effective method for underlay-aware overlay networks using Lightweight Directory Access Protocol (LDAP). ISPs maintain directory servers to provide their network topology information to overlay networks. By such an approach, overlay networks can know the underlay network effectively instead of sending probe packets. Furthermore we executed an experimentation to find that LDAP server can withstand frequent accesses like flash crowd and to measure behavior of a P2P application.