The thesis addresses shortages of the IPv6 security shield, provides comprehensive analyzes of IPv6 protocols, particularly mobility and security protocols, and identifies pitfalls of known IPv6 authentication means. Afterwards, it conducts a comparative study of existing authentication mechanisms and defines a list of requirements as a postulate for the proposal of new authentication mechanisms and methods. In order to propose the solutions which solve the identified shortages following the imposed requirements as close as possible, the thesis defines three subclasses of IPv6 environments ; the link-local, mobile and distributed environments. Following this it proposes authentication and authorization mechanisms and methods based on a combined and transitive opportunistic approach for each environment. With slight modifications, they can operate as a generic authentication mechanism or method in any IPv6 environment. For the purpose of the comparative study of authentication mechanisms and the formal specification and verification of the proposed solutions, the thesis proposes a formal modal logic as an extension of the Coffey-Saidha logic for verifying public key protocols. Finally, the thesis proposes an implementation framework, the extensible opportunistic IPv6 authentication framework which enables the straightforward implementation of the proposed authentication mechanisms and methods.