bridge? tunnel?

[Shawn Yarbrough <shawn@nailstorm.com>]

Networking theory question:

I have a Debian computer acting as a DSL router.  My ISP gave me 5
static IP addresses but I'm currently using only one, for the router
itself.

The rest of my network is masqueraded very successfully.

But what I want to do now is "promote" one or another of the masqed
computers onto the "real" static subnet given to me by my ISP.  Some of
these computers may be running Linux, others Windoze.

I think I understand that a bridge does this kind of thing for entire,
adjacent physical networks.  But I want to continue to masq some of the
computers on my network, while routing other computers the traffic from
my ISP-level subnet.  So the result is that I want two logical
IP networks running over my physical network, one private, masqed, and
unroutable, and the other public, static, and fully acting as part of
the internet and my ISP-level subnet.

Can bridging do this?  Or would IP tunnelling help me accomplish this?
Or is there a better way?  And how can I easily do this in Debian?

Thanks,

Shawn Yarbrough
shawn@nailstorm.com