There’s lots of good stuff on YouTube, including from David Bombal and Jeremy Cioara. If you’re more of a listening-while driving person, years ago the Security Now podcast did a “how the internet works” series that gives a terrific overview of the TCP/IP stack (it’s from 2006, but it’s still very applicable). And if you like to read, Michael Lucas just released a “Networking for Sysadmins” book, which is excellent.