Is it a true modem or is it a router... many people call the device provided by the ISP a modem when many times it is a router.
If it is a router then maybe.
If not then you have the issue of how do you share a single IP address with out the need to pass though a router. This is the key function must people need a router for.
It all most likely does not matter. I doubt you have a internet connection that is even 50m much less a gig. If you really do have a gig internet connection then a couple extra real ips is nothing compared to the many thousands of dollars a month you are paying already.