User mobility in wireless data networks is increasing because of technological advances, and the desire for voice and multimedia applications. These applications, however, require handoffs between base stations (or access points) to be fast to maintain the quality of the connections. In this paper, we introduce a novel data structure, the Neighbor Graph, which dynamically captures the mobility topology of the network, and we show how neighbor graphs can be utilized to reduce the authentication time of an IEEE 802.11 hand-off from 1.1 seconds (full EAP/TLS) to 50 ms without loss of security. (UMIACS-TR-2003-106)