I need to reset my computer's ip address to dhcp from static

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http://askubuntu.com – I have been running a file server on Ubuntu for a couple of years now. Recently I had to take the network down. When I brought it back up, my server lost its ip address from the router. I can't remember how to put it back in the dhcp mode. I know that running a server in the dhcp mode isn't optimal, but my router won't allow me to assign static ip addresses. It will let me set a "forever" lease time which has worked just fine. All I need to do is go dhcp on the server, find the ip address it has been assigned, and then set that as a static ip address on the server. Again, it's not t ()