If anyone is interested, I've got the following setup working:

ISP <---> ADSL modem <---> Hub
                            |
                         NetBSD box (Gateway)
                            |
                  |---------|----------|-------|
               MacOS     MacOS    Window-95   NetBSD

I have an internal network with multiple machines connected on an 
ethernet. Also on the ethernet is my ADSL modem for external network 
access.  I have IPNAT running to give me Internet (external) network 
access from all my internal machines through a single IP address 
dynamically assigned by my ISP using DCHP-client.  I can also access all 
my internal machines from any system on my inside network, including the 
NetBSD gateway box.  Access to the NetBSD box can be by internal name 
using my inside IP address (192.168.1.16) or by external name using my 
outside IP address (208.x.x.x).  Samba filesharing is working for Windows 
clients and NetAtalk filesharing is working for MacOS clients all on my 
inside network.  Apple File Sharing also works between MacOS systems.  
And to top it off I have the NetBSD box configured to serve out the IP 
addresses to my inside machines using DHCP-daemon.

So it is possible to do all this with a single ethernet connection on the 
NetBSD system. I'm sure it's obvious how to do this for someone who 
really understands this stuff, but it took quite a bit of head scratching 
on my part.  I think it's pretty simple to convert my setup to use 
multiple ethernet interfaces or a ppp link to the Internet world.  In 
fact prior to getting my ADSL line I used this on a ppp link.

I'd be happy to put all my configuration files together for others to 
start with if anyone thinks this would be useful.  I've also got some 
scattered notes about how to beat MacOS and Windows-95 into submission 
with this setup, and I have some old notes about how to connect to the 
NetBSD box using PPPD and SLIP which I've done with an old MacPlus system.

-bob