Abstract: Multicast and unicast communication among computing devices across different local area networks (LANs) and without static IP addresses is supported by assigning an instant-share (InS) address to an individual computing device. The InS address is recognizable by a dedicated router located in the Internet and enables the dedicated router to communicate with the individual computing device. The individual computing device embeds an InS address of a destination computing device in a data message to form an extended data message, and sends the extended data message to the dedicated router. The dedicated router then forwards the extended data message to the destination computing device. A group member contact synchronization among different computing devices in a group without Internet connectivity is also supported. Local lists of group members from different computing devices are collected. The most-recent one is used to update the local list of group members of a computing device.