In this class the technologies that make today's communication networks work are introduced and discussed. First the physical layer, which is responsible for an adequate transmission across a channel, is discussed briefly. The second part deals with the error control, flow control and medium access mechanisms of the data link layer. Finally the network layer is discussed. It comprises mainly routing and congestion control algorithms. The Internet is thoroughly studied throughout the class. The higher layers (transport, application) are subject of Communication Networks 2, which is the sequel to this class.