I have a newly installed Knoppix/Debian (on HD) and I made an apt update/upgrade via network.

When I booted the pc for the next time, I couldn't reach any any IP outside my network anymore - this is probably because there is no route:
#route shows:
192.168.1.0 * 255.255.255.0 U 0 0 0 eth0

My DSL-Router is 192.168.1.1 and I can ping it without any problem and also the other pc in the network.
But when I try (after: ifconfig eth0 down - and also up - I tried both)

route add default gw 192.168.1.1

I always get the message:
network is unreachable

The same message appears already during boot process.

Has anybody any hint what the problem is?
I would be really, really glad!