Trust and reputation systems for virtual communities are gaining increasing research attention. These systems track members' activities and obtain their reputation to improve the quality of member interactions and reduce the effect of fraudulent members. As virtual communities become a central playground for internet users, the reputation a member gains within a community may be viewed as a social credential. These credentials can serve the user as a means for promoting her status in new communities on one hand, and on the other hand assist virtual communities to broaden their knowledge about users with relatively low activity volume. The Cross-Community Reputation (CCR) model was designed for sharing reputation knowledge across communities. The model identifies the fundamental terms that are required for a meaningful sharing of reputation information between communities and proposes methods to make that information sharing feasible within the boundaries of users' and communities' policies. This paper presents the CR model and draws the architecture guidelines for designing an infrastructure to support it. The proposed model is evaluated by using a sample of real-world users' ratings as well as by conducting a dedicated experiment with real users. The results of the experimental evaluation demonstrate the effectiveness of the CCR model in various aspects.