From time to time I forget the release naming conventions for IOS/NX-OS software from Cisco.
I just happened to find this document today when I was trying to refresh my memory; explains what I needed QUITE well!
http://www.cisco.com/web/about/security/intelligence/ios-ref.html
This is always a great guide if you’re not too familiar with Cisco IOS (12) operations.
http://www.cisco.com/en/US/docs/ios/12_2/configfun/configuration/guide/fcfbook.pdf