It’s a bit different from what we’re used to do in Ubuntu, here are the instructions, assuming our network interface is eth0:

1. Take a backup from the “/etc/sysconfig/network-scripts/ifcfg-eth0” file, e.g. by the following command: “cp /etc/sysconfig/network-scripts/ifcfg-eth0 /etc/sysconfig/network-scripts/ifcfg-eth0.bak”
2. Open the “/etc/sysconfig/network-scripts/ifcfg-eth0” file for editing using vi or similar and do the following:
   1. change BOOTPROTO from “dhcp” to “static”
   2. Add the following lines replacing the “X” with your values:
      1. IPADDR=XXX.XXX.XXX.XXX
      2. NETMASK=XXX.XXX.XXX.XXX
      3. GATEWAY=XXX.XXX.XXX.XXX
      4. DNS1=XXX.XXX.XXX.XXX
         Note: You can add more “DNS1” entries to add additional DNS servers
3. Restart the network:
   “/etc/init.d/networking restart”

That’s it! Enjoy!

Source: http://www.howtoforge.com/linux-basics-set-a-static-ip-on-centos