This is usually found in the Advanced area of the router settings- where it is actually found varies by router brand. When you do find it you will need the PS3's MAC Address- you can find this in System Information in System Settings. Just put that in the blank provided, put in the IP address you want to use (making sure it is high enough up in the range of your router to avoid messing up with anything else going online) and hit the OK/ Apply/ Save Settings button.