Here are some simple steps to change Netmask for an IP in Linux from command prompt.
Login into your Linux box
From command prompt open the /etc/ips file which contains the IP configuration and netmask.
If you have many IP addresses then search for the IP address for which you wish to change the netmask.
Change the netmask, save the file and exit.
Now you want to restart the IP aliases as well, which you can do from following commands.
service ipaliases restart
Once done you can verify the change from following command
cat /etc/ips | grep 126.96.36.199
replace 188.8.131.52 with your IP Address for which you have changed the netmask.