Case resolving: ubuntu cannot reach internet
Ubuntu server cannot reach the internet - getting "Network is unreachable" error for any action that requires internet access.
Problem & solution:
in the /etc/network/interfaces file there were 2 missing lines in the eth0 configuration - the 'network' and the 'gateway' lines.
For example (the 2 missing lines in bold):
iface eth0 inet static
After editing the file (e.g. sudo vi /etc/network/interfaces), you should restart the network service (sudo /etc/init.d/networking restart)