A seamless vertical handoff method allows the network applications and connections on a mobile node to continue without disruption as it moves within a wireless overlay network that comprises multiple possibly overlapping layers of wireless networks (e.g., a WLAN and a WWAN) with different underlying technologies, providing mobile roaming capabilities. The method comprises a WLAN access point signal strength monitor for determining when to switch between WLAN and WWAN, and a network connection migration scheme that can move an active network connection from a wireless link of one technology to another wireless link of a different technology in a way that is transparent to the user, the remote end of the network connection, and the operator of the WWAN carrier.