Dynamips is a software that emulates Cisco IOS on a traditional PC. 

It has been created by Christophe Fillot who started his work in August 2005. 

The last official release of Dynamips supports Cisco 7200, 3600 series 
(3620, 3640 and 3660), 3700 series (3725, 3745), 2600 series (2610 to 2650XM, 2691)
and 1700 series. 
Please note that you must provide your own IOS images compatible with the Cisco hardware emulated by Dynamips.

GNS3 is based on Dynamips and Dynagen (a text-based front-end for Dynamips) to create a complete virtual Cisco
network, adding many additional features and most importantly making it easy to create, change and save your network topologies.

Unfortunately, Dynamips is no longer officially maintained, this is why we have chosen to make – with the precious
help of contributors – an unofficial version of Dynamips called dynamips-community that brings many improvements.
We encourage contributions and the code repository is accessible on GitHub.