The IOS release numbering convention is used to identify the release of the IOS software, including any bug fixes and new software features. An example of the numbering scheme is shown in the figure for both the mainline and T trains:

A single set of individual release numbers are used for all Cisco IOS Software 12.4 trains. Cisco IOS Software Maintenance Release 12.4 and Cisco IOS Software Release 12.4T use a pool of individual release numbers that are shared across the entire Cisco IOS Software 12.4 release family. Cisco IOS Software Release 12.4(6)T was followed by 12.4(7)T and 12.4(8)T. This permits the administrator to track changes introduced in the code.

Note: Any caveat that is fixed in a T train release should be implemented in the next mainline release.