Some cable ISP's, Comcast being one of them, only give out one IP address to one MAC address via DHCP. What this means is that if you can plug your computer in and it gets online, but if you swap in your router, the router won't get an IP. If you plug in your router and then powercycle the Cable Modem (unplug it, plug it back in to the power) It will clear that and again give an IP address to the first device it sees.