My PC's have static IP's and so must the Pi which has shipped dynamic in the config (DHCP)
need to change the config
first struggeling to get into the root - password unknown to me..
found out to change password with: sudo passwd root and entering my own password. note: some wrote that root originally had none, but it was impossible to enter no password at login.
Then tried to run MC to find/edit the config - BUT WHAAAT - MC NOT INSTALLED BY DEFAULT (see my other post about getting installed this)
Ok - into the graphical if with startx and consulting Debbian manual. The config is in in folder /etc/network and file interfaces
iface eth0 inet dhcp
iface eth0 inet static
and add below
needless to say that You must know what IP subnet and DNS server You are having