The course introduces basic elements of modern computer and telecommunication networks. The focus is on the four upper layers of the popular five-layer TCP/IP model. In each layer, the state- of-the-art hardware and software technologies are introduced. These include, for example, DNS, HTTP, SMTP, and P2P systems at the application layer, TCP/UDP protocols at the transport layer, routing and forwarding, intra-domain and inter-domain routing algorithms at the network layer, random access protocols at the MAC layer, and local area networks (Ethernet and WiFi). Advanced topics such as multipath TCP and software defined networking are also briefly discussed.