I'm not familiar with DIR-100. Is that a D-Link model?
Usually this is done by going to an internal address (the same as your default gateway) and entering a password. Like on my old Belkin, I'd go to http://192.168.1.1
and log in, then dig around on the settings and there'll be a button for the router to release its WAN IP address. Each router is a little different in their web interface and I can't be much more helpful than that, sorry.