I am having the following problem, and the setup is like this
I have a router which is sharing a 2mbps adsl connection to all of my clients, about 30 (MAC and PC) the router then jumps to a hub and then another hub. I also have a long cable about (30 mt or 90ft, whichever you prefer) and goes to an unmanaged switch. I have checked my cable with a tester and it works fine, I also plugged it it to one of the hubs and the light came on real quick, I thought everything was fine, however on the network that depends from the switch hanging from the 30 meter cable can't see my other computers or get an ip form the router!!!