Here’s how to setup a netmask while creating a zone:

# zonecfg -z testzone

testzone: No such zone configured

Use ‘create’ to begin configuring a new zone.

zonecfg:testzone> create

zonecfg:testzone> set zonepath=/zones/testzone

zonecfg:testzone> set autoboot=true

zonecfg:testzone> add net

zonecfg:testzone:net> set physical=hme0

zonecfg:testzone:net> set address=10.10.10.1/24

zonecfg:testzone:net> end

zonecfg:testzone> verify

zonecfg:testzone> exit

#

If you want to set a non default netmask you can replace that the following:

zonecfg:testzone:net> set address=10.10.10.1/24

Now you have set the netmark to be 255.255.255.0

There are various online tools to help you calculate the netmask number