Odd RedHat 9.0 behavior...
 

I'm altering the "/etc/sysconfig/networking/ifcfg-eth0" file. Change the IP address. Do "ifup eth0", it comes up. Do "ifconfig", it shows the IP I just set. When I ping something, traffic GOES on the wire, that is I see ARP requests and ARP replies, but the box does not pick up the ARP reply!

Then I restart the network service. Still nothing. Reboot. Works fine with new IP.

What am I doing wrong?

iptables
 

Do you have a firewall running? Try clearing your iptables with:

iptables -F
iptables -P INPUT ACCEPT
iptables -P FORWARD ACCEPT
iptables -P OUTPUT ACCEPT

I'm not running RH9 [I'm on Slackware 10] so can't check. Just a guess.

Yes iptables was running, but it was open all the time. The strangest thing is, it worked fine this morning. Changes IP instantly.

Before that, and that's happening on several different machines with RedHat 9.0 spanning months and countless reinstallation in a test environment, it took all sort of tricks to get the new IP going. Sometimes in addition to editing the ifcfg-eth0 had to run one or several of RedHat's IP changing programs. Then it worked. There was no rhyme or reason to what it did.