>Rick,
>
>1. We have 3 offices A, B & C. In office A, I can successfully map a network drive connecting to the server in office B, but fail in mapping a drive to the server in office C. (I can ping the server without problem) However, in office B, there is no problem mapping drives to the servers in both office A & C. What may cause the problem?
>
>2. Before I installed a broadband line in my home, I was able to map network drives to the servers of all offices A, B & C at home. After I have upgraded my 56K dialup to a broadband line. I failed in mapping a drive to any of the offices. However I can successfully ping the servers of all offices. What's the problem?

Some broadband ISPs disable custom TCP/IP ports. They open the most standard ones but block all others. You might want to check.