if anyone has idea about different between cisco NAC agent and cisco Clean Access Agent, please share your ideas.
In 4.6, the agent was overhauled and is now called the NAC agent. Previous versions were referred to as the Clean Access Agent. So pretty much, the 4.5 agent and 184.108.40.206 agents are Clean Access agents, and the 4.6.x and 4.7.x agents are called NAC agents.
Some of the changes made were moving a lot of the agent configuration to an XML file, redesigning the GUI, adding a service portion (so that the stub agent is no longer required), and better agent logging.