I ran into an issue where I replaced a network card in a computer and the Ubuntu installation would not  display the new network card when running the command ‘ifconfig’.  Running ifconfig would only display the loopback interface ‘lo’.  I found that the network card had been recognized by Ubuntu but that my interface configuration had not been updated automatically.

To correct this problem I had to perform two steps.

dmesg | grep -i “eth0″

This command let me know that the computer had renamed the new NIC to eth1, instead of the default eth0.  After I found the new name for the NIC, I had to update my interface configuration.

sudo nano /etc/network/interfaces

Once I open this file, I need to change any instance of eth0, to eth1.  Save the file by hitting ‘ctrl + x’, and then confirm by hitting ‘y’.

Restart the network service

sudo /etc/init.d/networking restart

That is it, your new card should now be displayed in ifconfig and have an IP.

Update:  This is also a common problem with Ubuntu based VM Appliances.  Pre-configured installations of Ubuntu could have this issue if the network card used in the installation is different from the NIC used in your VM software (ex. using a vmware virtual disk in a virtualbox virtual machine).