HI TO YOU ALL!!!
Eventually I find the way. I can login trought the ISPconfig interface!!!
I'm not sure of the step but I only comment out the line in the /etc/hosts:
127.0.0.1 localhost.localdomain localhost
192.168.100.1 server1.example.com server1
# The following lines are desirable for IPv6 capable hosts
::1 ip6-localhost ip6-loopback
..and after the restarting of the network my eth0 was configured as IPv4 having the right IP address.
(PS= I did it because in the perfect setup of Ubuntu 6.10 they said about that..)
I hope this long post can help somebody else!!