AbstractThis paper traces the evolution of LANs ad MANs from the early Ethernet days to recent Gigabit per second prototypes. Three generation are identified. The first generation includes Ethernets, Token Rings and Token Busses, and has a speed upper limit in the order of lOMbit/s. The second generation consists of architectures which have evolved from the first generation exploiting the fiber optics technology and properly modifying the access protocols. Higher speeds (in the order of hundreds of Mbit/s) and larger distances are thus achieved. Typical examples are FDDI and DQDB. The third generation represents a further step in the direction of higher speeds (hundreds of Gigabit per second). The key elements to achieve such high speeds are passive optic components and wavelength division.The paper consists of two parts. In the first part, the evolution of topologies and access protocols from generation to generation is described. In the second part, two representative examples of third generation networks, namely, Tree Net and WON, are presented.