Cisco IOS Version Guide

IOS version numbers are formatted as follows:x.y(z)t

The “x” designates a major revision number.
The “y” designates a minor revision number.
The “z” designates an individual release number
The “t” designates a train identifier.

Thus, the third release of IOS version 12.4 would be identified as 12.4(3).
The major and minor revision numbers combined is often called the
Maintenance Release number (for example, 12.4).
Trains identify IOS releases to specific markets and are represented by a
single letter:

The T or Technology train is continuously updated with new
features and security fixes.
The E or Enterprise train contains features and commands for
enterprise-level equipment.
The S or Service Provider train contains features and a
command-set for specific ISP equipment

The absence of a train identifier denotes a Mainline release. Security
updates are released for the mainline train, but new functionality is never
added to the feature set.
The latest version of the IOS (as of this writing) is 12.4(11) T. To view the
IOS version of your Cisco device:
Router# show version

The Cisco IOS is stored in Flash on Cisco routers and Catalyst switches, in
a . BIN file format. It can be upgraded using one of several methods:

Replacing the existing Flash stick
Via a TFTP server
Via Xmodem
Via a PCMCIA slot (not supported by all Cisco devices