My modem/router is a PK5001Z from CenturyLink. IIRC a tech support person told me that it uses ppp internally.
With regard to security, I would prefer to trust Windows or the modem/router as little as possible, hence the desire to connect the Windows box to the main box.
I would like to be able to manipulate the main box so that the Windows box is invisible to bad guys, i.e. has no global IP address. I would like to be able to manipulae the main box so that the Windows box cannot connect to the outside world, even through the main box. On exceptional occasions, e.g. updates, I would like to be able to manipulate the main box so that the Windows box can connect to the outside world.
I'm going to do some experiments before replying again.
BTW CableOne would give me more than one global IP address without paying extra. I discovered this playing around with virtual box, though I do not even have it installed at the moment.