I am in the process of setting up a Dell PowerEdge 2950 server with Red Hat Linux AS 4 with the 2.6 kernel. I had this server online and it was working fine. Today, I had to move it to a more secure subnet to replace an old computer. I took the network cable out of the old computer and plugged it into the 2950's eth1 port which where the original cable was. I also modified the various network-related files and rebooted. The problem is its, no longer on our network and I am stumped.

If I type "ifconfig eth0" I get

"eth0: error fetching interface information: Device not found"

When I look at the port on the computer, both lights on the network port are on, just like other Dell 2950s in the same rack that work. All I did was switch network cables and adjust the various network files such as /etc/sysconfig/network and /etc/hosts, not to mention
/etc/sysconfig/network-scripts/ifcfg-eth0

I also checked the /etc/modprobe.conf file but it is unchanged and its the same as a copy I made prior to this network change.

If anyone has any ideas, please let me know.

Are you using switching gear that has port security enabled?

Maybe you can try running 'kudzu' to detect hardware changes? I think it exists in redhat ...
This would do the same as a restart.

Also try ifconfig -a
to view all interfaces. Is eth0 listed there?

when I type ifconfig -a, I get just the local device, not eth0 or eth1. There are no restrictions on the switch either, which I just confirmed. If I type

ifconfig eth0

I get back

eth0: error fetching interface information: Device not found

Kudzo is set to run upon each reboot if need be, but it doesn't do anything to help this problem.

Check dmesg for interface related stuff. I'm not sure if have rebooted the server? (so you have run kudzu)?
Is the networkcable/switch on that end actually working? Plug in another device and test the connectivity.
Good luck!