internet + file sharing = HELP NEEDED! How should I setup my network settings?

I need help finding the correct settings here. I want a computer which uses a PPP internet connection on DSL, to also be able to do local file sharing to my laptop. Currently I can connect to the internet, or I can share files from my hard disk over my local network, but I can't do both at the same time.

Here's my setup and how I want things to function:

Code:

                connect to net via PPP DSL
                share files locally to Mac B 
                Mac A
 DSL modem         |             
          \        |        __________Mac B
           \       |       /          access Mac A's shared disk
            \      |      /           connect to net via neighbor's airport
              ethernet hub
              (aka switch)

Things I've tried:

1) When Mac A's network is set to have TCP/IP use PPP, then I can connect to the net via DSL, but it cannot do file sharing to Mac B.

2) When both Mac A and Mac B are set to have TCP/IP use DHCP, then Mac A can share its files to Mac B, but it cannot connect to the net via DSL.

3) If Mac A connects to the net via DSL using PPP and then I turn on file sharing, and then on Mac B I manually type in the address that Mac A's file sharing screen says it can be reached at. That results in Mac B connecting to Mac A's shared disk by networking over the internet! Which is obviously out of the question.

4) At some point (I forget what the settings were) I managed to have to Mac A sharing its files with Mac B and also had both connected to the net at the same time, but Mac B was connected to the net via Mac A's internet sharing. But my DSL connection is very slow, and having internet sharing turned on seemed to cut my bandwidth to each computer in half, regardless of if the other computer was actually sending or receiving any internet data or not. So that setup is unacceptable.

note: Mac B seems to be able to connect to the net via my neighbor's airport regardless of the network settings I've tried so far.


How the heck do I get this to work?! Is there some sort of submask or proxy setting I need to change? Help!

 

No, I have the setup diagramed above and my mac connects to the DSL with no problem. I'm using an ethernet switch, but I called it an ethernet hub also because that's how many people know it.

And even cooler is that with the setup diagramed above, any Mac on the network can be directly connected to the DSL net connection simultaneously rather than just having one Mac connected to the net and the other's just piggybacking via that first Mac's internet sharing services.

But I need to find a way to have them both connected to the net in a fast way and still have file sharing between them locally.

 

Oh, I think I see. By "act as a router" you mean share its files, correct? So you're saying it's not possible to have an ethernet cable going from Mac #1 that sends carries its internet connection and also sends file sharing traffic locally to another Mac in the room, all connected by an ethernet switch. Do I understand that correctly?

 

Mac A is getting its net connection over DSL via PPP.
Mac B is getting its net connection from a neighbor's wireless connection which is solely unrelated to the net connection that Mac A uses.

So these two separate computers are running their two separate net connections via two different ISPs and two different networking technologies.

At the same time I want Mac A to share the files on its hard drive with Mac B directly over my local ethernet connection through the ethernet switch.

 

I suppose a more precise diagram would look like this:

Code:

+ = file sharing data
- = internet traffic
((( = wireless



                
                Mac A
 DSL modem         |+                           
          \        |+          +++++ Mac B -----(((   airport   (((-----
           \       |+        +                        to separate
            \      |+      +                          net connection 
              ethernet switch