if you have many host connecting and diconnecting from your router you may get a different ip address each time you log on, but in reality if you only have a few hosts then the router will try and assign you the same ip address each time you log on, my box is setup using DHCP and always gets the address 192.168.0.6 and my computer always gets the same ip address too.
If you have not changed your username and password it should be
username: xbox
password: xbox
if you have changed it then you will be able to find it out from your dashboard depending on what dash you are running,
what dash are you running there ?