When you send a DM, your message travels through many steps from the app on your phone to the recipient’s app. How is it determined which nodes in the network your message passes through to reach the right destination? How can the same message be transmitted via fibre-optic cables, copper wires and radio waves? And how can we make sure that no one else can read it? In this mini lecture, you will learn about the fundamentals of the internet and network security. This mini lecture is in English.