Ubuntu 12.04 VM interface configuration

Hi anyone who can help,

I am not a very experienced Linux person, but I have managed to setup and get running a VM running on my ESXi box. I am having great problems with networking and the quick start guide doesn't include clear instructions on setting a fixed ip address. I have added an entry into /etc/networks/interfaces as follows:

# The loopback network interface
auto lo
iface lo inet loopback

# The primary network interface
auto eth0
iface eth0 inet static
        address 172.20.0.16
        mask 225.255.255.0
        network 172.20.0.0
        broadcast 172.20.0.255
        dns-nameservers 91.208.105.32 91.208.105.160
        gateway 172.20.0.254
        up route add default via 172.20.0.254
        up route add -net 172.20.0.0/24 gw 172.20.0.253
        up route add -net 192.168.0.0/24 gw 172.20.0.253

The trouble is that whenever I reboot all the routes are removed and the default route is set to 255.255.255.255. I have to set the interface to up again using the command in the guide:

sudo ifconfig eth0 172.20.0.16 netmask 255.255.255.0 up
sudo ip route add default via 172.20.0.254

Any help would be greatly appreciated. I have tried to find where the network is started and why the stack always goes into an attempt to set up a DHCP nic.

Many thank