Dynamic and Static IP in Ubuntu

Edit the following file:

/etc/network/interfaces 

for Dynamic IP:

auto eth0
iface eth0 inet dhcp

for Static IP:

auto eth0
iface eth0 inet static
      address 192.168.0.100
      netmask 255.255.255.0
      network 192.168.0.0
      broadcast 192.168.0.255
      gateway 192.168.0.1