Your comment is emblematic of the silliness prevalent on HN when it comes to studying from textbooks. Publishing date has nothing to do with core concepts in any field. Only pedagogy with an eye to clarity and appeal to intuition matters. All newer concepts are built over existing fundamental layers which is what one should study first. My recommendation (i have substantial experience in the area of network protocols and programming) is excellent in that regard; the only one that i know of which deals with different types of networks and the applicable theory (eg. queuing, scheduling, flow control etc.) for each.

The core Internet protocols (IP, TCP, UDP) haven't changed much, other than IPv6. We now have some amazing TCP refinements for congestion control and packet loss. In general, anything you learned about this stuff 30 years ago would still apply.

Most (all?) modern technology is built atop things that came before. Many texts like this one are still useful (although I can't speak to this specific book).

To understand a TCP connection, e.g., I imagine a book from the 1980s would be perfectly fine.