Here is what it says about the modem on this page's FAQ guide...
Since the SB4200 has both Ethernet and USB connections, can I use both to attach multiple computers?
Yes. The USB port will support only one user and the Ethernet port (with a hub or switch) can support multiple users, to a maximum of 31 attached computers. Please check with your ISP or Cable Operator to see how many users they allow on cable modems.
Since the webpage FAQ http://www.gi.com/noflash/sb_faq.html
mentions both the SB4100 and SB4200, I assume that this applies to both.