Hi,
I think this should be something as given in this post:-
https://supportforums.cisco.com/discussion/11700871/asa-sub-interface
Also Refer this:-
http://www.cisco.com/c/en/us/td/docs/security/asa/asa72/configuration/guide/conf_gd/intrface.html
Thanks and Regards,
Vibhor Amrodia