At home we have 5 machines (call them PC1, PC2, etc) that all share a workgroup.

A vonage voip router is attached to the cable modem and PC1 is attached to its first port, and an 8-port older linksys hub is attached to is second port. PC2-PC5 are on that hub. The result is that all 5 machines have internet access, however PC2 and PC3 cannot see the other machines, nor can anyone see them (in net neighborhood). Changing which ports on the hub PC2-PC5 are attached to does not change the status of PC2 and PC3.

But when I replace the hub with a router, now PC2-PC5 can all see each other, as they should.

Can anyone tell me why two machines would lose network visibility when attached to a hub as opposed to a router, yet retain internet access?

(note: PC1 and PC5 are winxp, PC2, PC3, and PC4 are win2k)

It could be due to a DHCP problem or a "uplink"-issue.

Some routers come with a "uplink"-port, meant to extend your network with other routers.
(Dunno if you can connect hubs to that port)

Having an uplink-port determines also if through each port multiple ip's (and/or services) can be assigned or not.

This limitation is usually only for the DHCP, and has no effect on static ip's.

So it might be a good test to try static ip's for all pc's.
(You'll have to enter the router's LAN-ip as gateway for all pc's, and grab your isp's DNS if you want them all to have internet access.

Using static ip's also allows you the use of a LMHOST-file.
For each pc in your network, make an entry in the file, like:

pc1-name "tab" pc1-ip
e.g.
PC-Patrick "tab" 192.168.1.30

Distribute the file over all pc's and look-up the file using the network properties dialog under advanced.

the file is typically stored under:
"C:\%windir%\system32\drivers\etc\"

Hope this helped,
Peter.