What?
here is the thing... if you have stitically defined an IP on a machine, it won't look to the router for on. so the router will be stupid to it. So long as the ip subnet is the same as the subnet used on the DHCP config from the router, it will talk.
now, to find out what the IP is, simple open a command window and type IPCONFIG /all This will tell you if the machine is dynamic or static and what the ip is for it's self. Also check the gateway. this should be set to the IP address of the router (Internal, not external).
BTW, this if ro windows so if your running something else, say so...
oh, and just to clarify, if a machine is set static, then another is set the same, the IP will only be visable on the machine. the router will not see it, as it doesn't care if a machine of this address or that address tries to talk though it, as long as the subnet is the same. except when two machine having the same address try to talk though it. It can only talk to one of them successfully.
hope that helps...
