An authentication protocol is a sequence of message exchanges between entities that either distributes secrets to some of those principals or allows the use of some secret to be recognized .Distributed Network, such as sensor and mobile ad hoc networks, must conquer a numerous of security challenges to realize their potential in both civil and military applications. Usually, a Distributed Network like ad hoc networks are deployed in untrusted environments. Therefore, authentication is a pioneer to any secure interactions in these networks. Recently, v a r i o u s a u t h e n t i c a t i o n p r o t o c o l s h a ve b e e n proposed for ad hoc networks. In distributed authentication services in ad hoc networks. Two nodes authenticate each other via signed, unforgeable certificates released by a ”virtual“ trusted certification authority. Compared with regular network authentication solutions [28, 29] that rely on physically present, third-party trusted (certification authority) server(s), our design takes a self organized securing approach, in which multiple nodes (say, k) collaboratively serve the role of a certification authority server. Therefore, the authority and functionality of the authentication server are spread to each node’s locality. Any local k nodes are trusted as a whole and collaboratively provide authentication services. This paper describes the authentication procedure and a classification that clarifies similarities and differences among authentication protocol described in the literature. The classification is based upon the role of nodes in the authentication function, establishment of certificate, and type of certificate.