You'd have to give a few more "hints"! i.e. what kind of modem do you use (USB or ethernet)? do you know whether your service is using a dynamic IP address (one that can change) or a static one (one that stays the same)?
Thats the kind of stuff that you'd need to supply e.g. my system uses a "modem/router" device, so that once it's set up for my connection it doesn't matter about the actual service, it stays as it is (static IP from service provider etc). So that all I need to know is the range of the internal IP addresses that the modem uses (something like 10.0.0.1 for the main "gateway"). I'd then just have to set up the LAN settings and use an internal IP of say, 10.0.0.2 and put the 10.0.0.1 into the "box" for the default gateway.
Which means the main pc using *.2 talks to the modem/router which is *.1 via the ethernet card/cable. Internally the modem/router then connects it to the static address that my ISP provides for my service.
The only difference, would be that most ISP's will provide a basic "dynamic" service, so in that case, the *.1 internal connection will not know what the external IP address from ISP is, but it talks to the DHCP server inside the modem/router, which will detect whatever IP number is being provided by the ISP.
Erm, yes, sorry. It does sound complicated when explained that way. It's not really (just sounds worse than it is).
It's different with USB modems though - IMO ethernet is better (and I understand faster).