Took me two days to figure this out. I dont know if I am just thinking wrong but in my searching, lots of others getting same error trying to setup dhcp in webmin. "bad subnet number/mask combination"
Problem is on the cfg page it askes for network address. I assumed that means the address of the eth0. But what thats actually asking for is the subnet 192.168.1.0. Which normaly has to end in zero. Changing 'network address" in the webmin cfg page changes the subnet in the actual dhcp3 file. So the naming of that input line really confused me and I think should match what its called in the cfg file.
Tried attaching pic but it didnt show up. So here's link to it. https://hostinglz.com/?p=163