[CentOS] netmask set with ifconfig doesn't hold

I gather that ifconfig is a way of setting the netmask in the current shell instead of a persistent value.  I say this because I am running it and see it for my specific network interface, directly after running it. However if I restart the network service the netmask is reverted to the previous value. What is the correct way to set the new netmask for the network interface to make it persistent.   if it's different for CentOS 6 and 7 I'd like to know that too. FYI - I tried searching about this but none of the info has really helped so far.  Also consider this like "changing netmask for dummies" since I am not an administrator and am just searching and trying these things. Thanks very much in advance.KM