Well, yes, I am saying BT here, because it has been confirmed very very clearly by BT that you should not have more than one PC on a single line. The exact quote is :-
"Openreach engineer has confirmed the same we cannot add multiple PC's to one single circuit as it causes instability."
Initially this appeared as just an engineer note relating to a long 20CN line. We naturally assumed it was just a silly comment, but a team leader confirmed it, initially relating to all 20CN lines, and then apparently checked with Openreach to confirm it and it seems to be any single circuit. It seems it is clear official BT policy that having more than one PC on a single circuit causes instability.
Obviously, from a technical point of view, we cannot see how this could "cause instability" - that simply makes no sense. Well, the whole statement makes no sense. We thought we bought a service that transfers packets (PPP frames) and not something that had strange restrictions on number of PCs. What this means is that if you do have more than one PC connected and an unstable line BT will refuse to fix the fault.
I suspect none of BTs other wholesale or retail customers are aware of this limitation. I would say we'd like a response from BT on this, but given we have asked several times for them to confirm they really mean this, and received the above quote, I am not sure what else these is to say.
I suppose it does mean there is no need for NAT anymore (yay!).
Odd that 10 years ago when BT launched their original broadband services (500K, 1M and 2M) with BT provided routers we were able to order routers with blocks of IPs configured to allow multiple PCs. Strange that BT have introduced this new restriction now.