Resolved

When i put the ip address in a new interface, and the new ip address is 10.x.x.x, it automatically put a /8 mask when i apply, even when i specified a /24 mask before.

This can create a big conflict when you have the same 10.x.x.x on another interface.

how to reproduce the bug :

take a factory default config and try to change the lan interface to 10.x.x.x

then push down the apply buttun.

Yes, you have to specify the subnet address before specifying the subnet mask.

In pfsense, the code that automatically selects the mask was just completely removed to avoid this problem, and it defaults to /24. Might make sense to do the same in a future m0n0wall release to avoid this problem.