On a computer connected to the same network do the following.
Start -> run/search -> "cmd" (without the " ) hit enter.
In the black box type "ipconfig" (again without the " ) hit enter.
It should give you all the information you need.
Normally its dns/gateway 192.168.0.1 or 192.168.1.1 and mask 255.255.255.0.
Your ISP probably has specific ones they recommend using, those should be available on their support site. However at times these do not function properly with all devices. I have had luck using open DNS servers (which are free to use) when using an older router and setting my Xbox in a DMZ, they are:
208.67.222.222
208.67.220.220
Of course when using these DNS servers you still need your assigned IP, Subnet and Gateway addresses assigned by your router/ISP.