How to add a cable moden to my network running windows server 2003

I’m running a windows server 2003 as file server, domain controller, dns and dhcp, my network range of ip is 10.0.0.xx everything works fine, I had a adsl modem and i were able to disable dhcp and enter my own ip settings on it, now I got a cable modem with a static ip 206.126.120.xx, how do i merge this with my network so all my 25 pc and server can have internet??