Access control is one of the core functions of any security system. An access control mechanism differentiates between authorized and unauthorized system use, based on a set of security labels such as those contained in an access control list (ACL).
There are several different models for access control, based on the level of centralization used in allocating system rights and permissions. Some of these models include:
Network Access Control (NAC) is a technical approach to policy enforcement with respect to client devices attaching to a network. NAC can be used to implement any of the models above.
Reference: http://www.techotopia.com/index.php/Mandatory,_Discretionary,_Role_and_Rule_Based_Access_Control