There is some ways that on a CentOS system you can set IP addresses staticlly, I mention two of these:
1) using command-line:
/sbin/ifconfig eth1 192.168.30.40 netmask 255.255.255.0 broadcast 192.168.30.255
in this command you can set your intended static ip address in place of 192.168.30.40, netmask in place of 255.255.255.0 and broadcast in place of 192.168.30.255 .
enter the following command as root:
now you see a window, select the interface you want to set a static IP to it (e.g. eth0) then press the "Edit" key. now another window will open, in this one select "Staticlly Set IP Addresses" radio button, then fill "IP address", "Subnet Mask", "Default Gateway Address" fields as you want. then press OK to save changes.