A netmask setting is not available when creating a zone, and the /etc/netmasks file is not consulted when plumbing up zone interfaces. This tech-recipe describes how to force a netmask when creating a zone.
To configure an IP address of 10.1.1.12 with a class C subnet mask (255.255.255.0, a 24 bit mask) on physical interface bge0, use these commands when running zonecfg to create the zone. (Substitute your IP address, netmask length, and physical interface as appropriate.):
. . .
This configuration is only necessary when using a non-standard netmask.