Edit /etc/network/interfaces
Change
<strong><em>iface eth0 inet dhcp</em></strong> to <strong><em>iface eth0 inet static</em></strong> and below enter <strong>address 192.168.100.1</strong> <strong>netmask 255.255.255.0</strong> <strong>network 192.168.100.0</strong> <strong>broadcast 192.168.100.255</strong> <strong>gateway 192.168.100.254</strong>