Abstract: The interconnection of LANs, its theory, method and key technique are discussed. On the basis of describing the theory, method of interconnection of LANs and its fealture, this paper discusses the internet architecture model, and analyzes the ISO, PUP and DOD internet architecture model. The internet protocol functions which include interact addressing, fragmentation and reassembly, routing, flow and congestion control, security and error check are presented. The datagram and virtual circuit service are evalutated. The gateway and the protocol translation are discussed.