Satellites offer advantage in their wide coverage and immunity from terrestrial disasters. Recently, the power of low earth orbit (LEO) satellites has been clearly demonstrated in many applications including communication and Earth observation. In addition, technical advancements are being made in both the space-grade hardware technologies and future network architecture. In this paper, an overview of network architecture and core technologies for LEO mega-constellation networks is presented, including onboard computing, optical inter-satellite link (ISL) communications, and network slicing. The state-of-the-art R&D efforts and future directions will be highlighted.