In CentOS 6x the IP settings are held in file(s) located in the /etc/sysconfig/network-interfaces directory, there is one file for each physical interface, they are generally called:

ifcfg-eth0
ifcfg-eth1
ifcfg-eth2
ifcfg-eth3
ifcfg-eth4
ifcfg-eth5
ifcfg-lo


To see exactly what files you have, and what they are called simply do a list

      ls /etc/sysconfig/network-scripts

To edit a file enter:

      vi /etc/sysconfig/network-scripts/ifcfg-eht(0) --- (use the correct eth number for your system)

Below is an example of a static ip configuration for the first NIC (ifcfg-eth0)

      DEVICE="eth0"\\ 
      BOOTPROTO=none\\ 
      IPV6INIT=no\\ 
      NM_CONTROLLED=no\\ 
      ONBOOT="yes"\\ 
      TYPE="Ethernet"\\ 
      IPADDR=192.168.27.110\\ 
      NETMASK=255.255.255.0\\ 
      GATEWAY=192.168.27.254\\ 
      PERSISTENT_DHCLIENT=yes\\ 
      DHCP_HOSTNAME=h64.envivio.com