Here's how you can setup a netmask in a zone. It's not difficult at all...!
#zonecfg -z zonename
> add net
net> set physical=bge134001
net> set address=192.168.1.100/24 (Here's the trick! You'll have to put it this way /24 in order it to work)
net> set defrouter=192.168.1.1
And then you have to reboot the zone:
#zoneadm -z zonename reboot