How to gain access to a routers settings?

At my college they gave us modems for internet connections. I have been trying to use my xbox to go on xbox live but it keeps failing on mtu settings. I tried to enter the external ip address but it keeps coming up as a wrong page. I know you can gain access to it because the name of the network isn't exactly something the school would put as the name of the router (your a ***) but I can't find out how to access the mtu setting. my router is a cisco 575 LRE.what is the usual ex. ip address for this? how would I get access to the router? Any tips at all would be great