Video presentation: Computer Networks and the Internet: the network core. Core network functions, packet swtiching, circuit switching, the Internet: a network of networks.
Computer networks class.

Jim Kurose
Textbook reading: Section 1.3, Computer Networking: a TopDown Approach (8th edition), J.F. Kurose, K.W. Ross, Pearson, 2020.
See http://gaia.cs.umass.edu/kurose_ross for more open student resources.