ipconfig/release: deassing current assigned ip address
ipconfig/renew: ask new ip address from dhcp server (=router)
release happens after 24 hours automatically and something 7 days. It depends how dhcp server has been configured. A few minutes before expiration he asks to extend the duration of that address automatically.
So if system powers off all of a sudden the ip address will be freed when expired on dhcp server.
in command prompt:
Delete ip.txt afterwards