George, Comcast, like all(?) DOCSIS systems uses 10/8 or one of the other defined non-routable blocks for cable modems, which (if its a DOCSIS certified device) will be a bridge only and will not do NAT. If you we're NAT'ed on a cable modem system it must have been a proprietary system, of which there once was a ton before DOCSIS caught on, that Comcast hadn't phased out. I don't believe that any of the large MSO's (and none of the small ones I know) are doing NAT on edge devices or the core at this point, however your point is still valid since virtually all of the ADSL lines deployed are being NAT'ed at the modem level and that means that millions of people are being NAT'ed without a choice. That also means that a lot of people are already going through two NAT translations since they have plugged in a small router behind their DSL modem and both are NAT'ing their buggy little hearts out. We also have to think about the tremendous number of people on all kinds of networks that are voluntarily, like me, NAT'ing through one device of their own (usually not educated) choice.
How many instances of 10/8 did they say they were running? I was behind a NAT when I had Comcast service. I am behind a NAT currently with my AT&T service.
Note that the NAT was done on the "cable modem" in both cases and not further in to their network. That said, from my reading of some industry large scale NAT devices (the A10 AX series is one which I am familiar with), they do things such as full cone NAT so it will still work with many applications that might break with conventional overload dynamic NAT.
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