how to change my ip address in centos 5

if your interface is eth0 you need to modify the file:
/etc/sysconfig/network-scripts/ifcfg-eth0

See this link for reference:
http://www.centos.org/docs/5/html/Deployment_Guide-en-US/ch-networkscripts.html

use setup command to control the centos system setting
and selected Network Configuration
Gud luck

setup
(or) use
system-config-network if you are using GUI
(or) use
system-config-network-tui

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The broadcast address has the last octet set to 255 only if your netmask is 255.255.255.0. This is usually the case for home users (usually a class C address), but often not the case in a larger network (such as a class B or a subset thereof).

If you understand bitmasks, logical AND, etc. you can calculate the broadcast address. It's not hard. But it's even easier to reference one of the websites on the Internet that'll do it for you such as:

http://www.subnetmask.info/