This post shows how to change the network mask and other ifconfig information on centos. After login to centos run the following command to open and edit the ifcfg-eth0 file:
Look for the line starting with PREFIX0, change the network mask here (not that this is specified in bits so, "PREFIX0=22" means network mask is 255.255.255.252).
After change is completed, restart the network service by running the following command:
service network restart