No network connection in debian.
 

I installed debian squeeze without any network connection.Later ,I added a modem But it does not work in debian.If you manuall use dhcp -v eth0 internet works.It does not connect automatically on startup.Any help is welcome.Previously I installed with the same modem and internet connection.It is automatically configured.
So,How to make et0 auto configured on startup?

You an use dhcpclient to start network at boot
http://wiki.debian.org/DHCP_Client

Do you use network manager or wicd? If NOT, add the following in your /etc/network/interfaces
The first line is to set eth0 take ip and the rest from dhcp and the second line is to start the interface when the network service starts, i.e. on boot.