My apologies for just taking your comment as a jumping-off point, but could you or anyone else point me in the best (free) direction to properly learn about thi

My apologies for just taking your comment as a jumping-off point, but could you or anyone else point me in the best (free) direction to properly learn about this whole stack of protocols? It's been very high on my list to finally figure out what sits below HTTP, and in a roundabout way my appetite has been whetted by reading a Claude Shannon biography and The Information (which I suppose aren't perhaps /that/ close to UDP and so on but more about information theory and error correction as a historical development), but those books made me really want to dive in and learn about the building blocks of 'the internet'. Any pointers to books, articles or courses would be very much appreciated!