I have a very specific problem with trying to set up a home network -
in spite of 2 nights, several books and countless hours online, I
can't crack it.

[reasons for why my test network is set up this way have to do with
cabling in my apartment]

1. I have 2 PC's, both running Windows XP
2. My apartment building has a LAN setup already, with a CAT5 wall
socket connection.  I plug the WAN port of my Linksys WRT54GX wireless
router into this socket.
3. I feed a cable from Switch port 1 on the back of the WRT54GX to an SMC switch
4. I feed a second cable from Switch port 2 on the back of the WRT54GX to PC1
5. I plug PC2 into the first port on the SMC switch.

From internet-sharing standpoint, everything works.  My router and
switch automatically get IP addresses and I can ping the router,
switch, PC1 and PC2 from either PC.

I am sure I am missing some key detail here, but stats are:

Router
IP: 192.168.1.1
Subnet Mask: 255.255.255.0
DHCP assignable IP's: 192.168.1.100-109

Switch
IP: 192.168.2.1
Subnet Mask: 255.255.255.0
DHCP assignable IP's: 192.168.2.150-199

What I want to do, however, is to use Windows networking to share files.

1. When I plug PC2 into the WRT54GX (i.e. dont use switch), I can see
PC1 and vice-versa (unique PC name and same workgroup name set up
correctly)
2. When I use the switch, however, the PC's cant see each other

From trawling the web, I've figured out that the reason is that the
router and connected PC are on a different subnet from the switch+PC 
(192.168.1.x vs. 192.168.2.x).  The problem is that everything I try
to fix that fails to work e.g.

1. Configure the SMC switch to use 192.168.2.1 and connected PC's to
use 192.168.1.x  The idea is for all connected PC's to have
192.168.1.x with care taken to make sure there is no overlap in IP
handed out by the router for PC1 and the switch for PC2.  This isnt
possible because a) if you force SMC switch to use x.x.2.x it is not
permitted to assign x.x.1.x,  b) note that the subnet mask is
hard-coded to be 255.255.255.0

2. Change the subnet mask on the SMC to be 255.255.0.0   I actually
dont know exactly how subnet masks work, but my guess is that making
this change would mean that x.x.1.x and x.x.2.x are now deemed to be
part of the same subnet.  It is actually not clear to me whether this
change should be made on the SMC or the WRT54GX but, in any case, SMC
setup screen doesnt permit this change to the subnet mask.  On the
WRT54GX similarly it is not possible to change the third 255 to a 0.

3. Lastly, I tried giving the SMC switch 192.168.1.2 (for example) vs.
192.168.1.1 for the WRT54GX.  Again, this doesnt work.  It seems to
case either or both of the units to "hang"

My question is, broadly, given the setup above how can I get Windows
file sharing to work between PC1 and PC2

I think you were closest with #3. 

1. *All* devices need to be on the same network (192.168.1.x).
2. *All* devices need same subnet mask (255.255.255.0). You said in #2
you changed  it. Did you keep it this way for #3 or change it back to
255.255.255.0?
3. Set only the Linksys router or the SMC switch to manage DHCP - NOT BOTH.

This ought to fix your problem. If, after this, both systems can see
each other but not share files, then you might be having a permissions
issue, not a networking issue.

Post back here with results.

JtB()

Hi Jebediah - Thanks very much for the prompt response.  I am on a
business trip at the moment, back home on Wednesday.  I will try your
suggestion below then and let you know how I get on.  Many thanks,
Jason

I agree that your biggest problem is that the switch is also handing
out DHCP.  Which to me sounds more like something a router would to
than a switch.  You'll want your Linksys to handle all the DCHP
addressing.  If you switch is manageable then I reccomend you give it
a static address on the same network as the linksys.  I would set it
to 192.168.1.2 since this is ouside of the IP scope your router will
hand out to your PC's.  Then give it the subnet mask of 255.255.255.0
and default gateway of the router (192.168.1.1).  At this point you
are not providing addresses for multiple subnets.

You can think of a subnet as a way of defining a local network.  A
very common subnet for home networks is the 24 bit subnet which is
255.255.255.0.  What this really does is it tells all computers with
this subnet that the first 3 octets (first three groups of numbers in
your ip address - 192.168.1) are on your local network.  In order for
a computer to see a machine on any other network (e.g. 192.168.2.x)
the computer will try to look at it's default gateway to find such a
machine.

Option 1 just won't work, as you've seen.

Option 2 would have worked for you only if all machines had a subnet
of 255.255.0.0 because then all the computers know that they are on
the same local network and not to send packets to their default
gateways based on the subnet mask definition.

Option 3, I dont really understand why this would cause both units to
hang.  This should be the best bet.  If the SMC is truely a switch
then the only reason it needs an IP address is for management.  You
should turn off all options in it for routing, dhcp server, and any
firewall settings.  Also if there is a WAN port on this switch don't
use it.  If one says uplink that is what you want to plug into the
router.  If you absoutely need to manage the switch after turning
these settings off try to give it an address of 192.l68.1.254 and
subnet of 255.255.255.0.

Hi Jebediah and Ericthegreatone -

Sorry to keep you both waiting.  I havent had a chance to look at this
again as my wife and I are now on a 2-week year-end holiday, wont be
back home until Jan 4.  I dont know how the protocol works in these
cases since this is my first Google question - how and when do you get
paid for providing an answer?  Incidentally, I did check on the day
before going on vacation and my SMC switch is actually a router.  I
know that falls in the "doh" category but I inherited the box and
didnt check.  I guess I will still be able to follow the approach of
disabling DHCP etc.  Will let you know when I am back home.
Thanks and best wishes for the holidays, Jason