I do ... software development? ;-) There isn't really any particular category for what I do, though I tend to do more of the lower-level/backend stuff of projects, and knowing how the stuff that you build on works internally certainly is useful in optimizing and debugging.
As for getting an understanding of how TCP/IP works, I think Stevens' classic TCP/IP Illustrated still is a good book to get started, even if somewhat dated in some details (no IPv6, in particular), but the general principles still apply. Though maybe there are newer equally good books around that I just don't know about?
Well, yeah, it's a book by the same name, but is it the same quality (well, it's not just the same name, of course, but a new author obviously can make a big change in quality, in either direction ;-)?
In any case, my recommendation was referring to the old edition by Stevens alone, no clue about the new one, though at least the newly covered material seems appropriate to me.
(For anyone who might not be aware: W. Richard Stevens died in 1999, so the new edition is by a different author, though apparently based on the old material.)
I have not worked through the entirety of the original, which I also own, but the new one seems pretty good to me so far. But you may have a different opinion as somebody who knows much more than me about TCP/IP.
As for getting an understanding of how TCP/IP works, I think Stevens' classic TCP/IP Illustrated still is a good book to get started, even if somewhat dated in some details (no IPv6, in particular), but the general principles still apply. Though maybe there are newer equally good books around that I just don't know about?