Love I.T., Live I.T., Embrace I.T.

For Red Hat distributions, you will edit:
/etc/sysconfig/network-scripts/ifcfg-eth#

For Debian distributions, you will edit:
/etc/network/interfaces

If you are running DHCP, or Static and want to swap between the settings, here are two examples for your use:

DHCP will look something like this:
DEVICE=eth0
BOOTPROTO=dhcp
HWADDR=00:D0:B7:08:09:BB
ONBOOT=yes

STATIC will look something like this: