If you are running Dell PowerEdge 2950 server or HP ProLiant BL480c G1 server and upgraded from old version to latest Red Hat Enterprise Linux ES release 4.5 (Nahant Update 5) you will notice that ordering of your NIC devices changed.

Fix problem before upgrade:

Before running upgrade via up2date command open /etc/sysconfig/network-scripts/ifcfg-eth0 file (for NIC 0) and add line
HWADDR=My-MAC-ADDRESS

Now proceed to upgrade using up2date -u
# up2date -u

Howto fix problem after upgrade:

However chances are very high that after running up2date -u command (i.e. after upgrade) you may notice the problem. Don’t panic. Open grub.conf file:
# vi /etc/grub.conf

Find out current kernel line:
kernel /vmlinuz-2.6.9-55.ELsmp ro root=LABEL=/ console=tty0 console=ttyS1,19200n8

Replace as follows:
kernel /vmlinuz-2.6.9-55.ELsmp ro root=LABEL=/ console=tty0 console=ttyS1,19200n8 pci=nobfsort

The pci=nobfsort option restore the original Kernel 2.6 ordering.

A final note

This problem is not Redhat specific. It can occurred on Suse or any other Linux distribution. Please see Linux enumeration of NICs solution published by Dell for more information.