The function of an authentication protocol in network security is to ensure that only authorized users gain access to a network or system. Authentication protocols validate user identities by requiring credentials, such as usernames and passwords, biometric data, or security tokens. They establish trust by using various methods, including encryption, to protect data during transmission. Common authentication protocols include Kerberos, OAuth, and SAML. By implementing these protocols, organizations can safeguard sensitive information against unauthorized access and maintain the integrity of their systems.