Ok this one of those tricky question and answer is use ifconfig command:
Display current IP address
Code:
ifconfig eth0
ifconfig -a
Set new IP to 192.168.1.10
Code:
ifconfig eth0 192.168.1.10
Set new IP to 192.168.1.10 with netmask
Code:
ifconfig eth0 192.168.1.10 netmask 255.255.255.0 up
See A detailed and step by step howto :
Howto Red hat enterprise Linux 5 configure the network card | Frequently Asked Questions