I connect to internet via my dorm's ethernet and IP addresses are assigned by DHCP. I tried
sudo dhclient -r dhclient
but that doesn't work. are there another commands i can issue?
p.s: I don't want to get a static IP, just get a new one.
It is entirely up to the DHCP server. It probably will give you (the MAC address owner) the same address as long as the lease has not yet passed its expiration time.
Make sure your /etc/network/interfaces file has a line similar to this:
Use this command to restart networking (and request DHCP IP in the process.)
You can also use this command to request DHCP: Ubuntu Request DHCP