Change netmask with /etc/rc.d/network restart or reboot the machine?

[Glen Barber]

You could change the interface settings in rc.conf, and
/etc/rc.d/netif <interface> restart.  I've added/removed aliased
interfaces in a similar fashion, and haven't experienced hiccups.

You should, however, double check your service applications that
listen on particular IP addresses to make sure they are going to
behave after netif restarts, as well as after a reboot.  Certain
changes, for example apache, require the service to be restarted as
well.

Regards.
-- 
Glen Barber