Change network mask manually

[xsynek@volny.cz]

Hi,
I have Zentyal version 5.0.
When I am changing network mask on interface by administration page I get error: Network interface is empty.
What is wrong? Is it possible to make mask change directly on some config file?
I know that /etc/network/interfaces is overwriten by each restart. I check network MAS file, but there is only script and I can't find where is data source for this script.
Can some help me?

BR Petr