The first thing we need to do, is open our interfaces file.  Execute the following from a terminal.

sudo vi /etc/network/interfaces

or

sudo nano /etc/network/interfaces

Then add or copy and paste the following.

auto eth0
iface eth0 inet static
        address 192.168.1.100
        netmask 255.255.255.0
        network 192.168.1.0
        broadcast 192.168.1.255
        gateway 192.168.1.1

Close the file. Now we need to change the DNS settings

sudo vi /etc/resolv.conf

or

sudo nano /etc/resolv.conf

Edit the nameserver line to something like the following.

nameserver 8.8.8.8

Close the file and then restart the networking stuff.

sudo /etc/init.d/networking restart