What is the difference between the CISCO iOS 12 and iOS 15 versions?

Cisco IOS is a software program. It can be used to route and telecommunicate. The developer is the Cisco Systems and was released on July 28, 2017. The functioning of Cisco is based on commands. This operating system is used through a multitasking system. There are about six command modes. Some of these include the ROM Monitor Mode and User EXEC Mode. In the IOS version 12.(20)T1, it can be broken down.

Twelve represents the major version number and four represents the minor version number. The number in parentheses, 20, represents the release number and one represents the software release train identifier. The main different depends on the where the numbers and letters are placed in the line following the version.