In my case, CenturyLink installs optical fiber all the way into my house, and terminates it with an ONT at my POP in my basement. My first clue to what they are doing is that they offer a "Modem", which is really just a standard ethernet gateway with only 10/100/1000 mbps wired ports, and no optical ports at all. Standard hardware you can buy anywhere (and which I already have in place with my Netgear mesh system)
They "claim" that the 940 mbps bandwidth is individual bandwidth, and not "shared" by neighbors ... but I would have to try it to know whether they are really delivering anything close to that.
last time I tried running NAS units in series it was a total mess. Never again
I dont know .... I have some soul searching to do. I originally told the door-knockers trying to sell me this that I was after better customer service than I had been getting from Comcast. Definitely not the case here - I could not get a single technical answer out of them. And the fact they call the C4000 a "modem" chaps my .... you-know-what. On the other hand, If I can really get 900+ mbps bandwidth, that would be .... cool to see.
(in one single series of exchanges here I got much, much, more clear discussion than several hours of phone calls (and sitting on hold) than I got from them - which is why I came here. Mostly this has solidified alot of what I was able to glean from research, but there is new info also. Thank you again to all!)