How do I switch my modem from a public IP?

A while back I was trying to open up my NAT type in my modem settings. Someone suggested that I enable the option to use a public IP address and I did. This did open up my NAT but I believe it is also the cause of my current problem, which is that I can't access my modem settings. My IP address is now something that starts with a 74. Any time I put it into my address bar it just takes me to a google page full of things like lists of IP addresses. Can someone please tell me how to either access my modem settings or to disable public IP, if that's even my problem. By the way, no I don't have a router, I have an ethernet cable running straight from my modem into my computer which I just alternate between my computer and xbox.