Probably can be identified as such by the unique MAC number that manufacturers hard-code into networking devices such as NICs, routers, modems, etc. Whether an ISP would go through the trouble of doing this with all connecting MAC numbers is another matter. If, instead of a hardware router, you connect and share access with a second NIC and sharing software, the MAC number that connects would be a standard NIC, and there is no way to tell how many computers are getting access through the server (unless they could get access to your computer and examine the registry, which you could prevent by unbinding Client for MS Networks and file sharing from the internet NIC, and installing a good firewall).