When I ran "/sbin/ifup", or "/sbin/ifdown", or "/etc/init.d/networking restart", I got some error message like

Ignoring unknown interface eth0=eth0.
Ignoring unknown interface eth1=eth1.
Ignoring unknown interface eth2=eth2.
Ignoring unknown interface ath0=ath0.
Ignoring unknown interface wlan0=wlan0.

Note that I do have eth0, and "ifconfig" shows that it is up and has been assigned an IP.

To fix the error, check the file /etc/network/interfaces, comment out the unwanted interfaces, and keep at least the following:

auto lo
iface lo inet loopback

auto eth0
iface eth0 inet dhcp