after preseed install the network does not work

When I do a network install on 12.04 via preseed the network do not work when I startup after the install has finished. The problem is that the interfaces file looks like this:

# The loopback network interface
auto lo
iface lo inet loopback

# The primary network interface
auto eth0
#NetworkManager#iface eth0 inet dhcp
# This is an autoconfigured IPv6 interface
iface eth0 inet6 auto

I get ipv6 adresses but I want ipv4 adresses as well.