I study a network based mechanism of norm enforcement in a community where agents play a repeated Prisoner's Dilemma with changing partners. Each agent chooses a number of close friends to whom he communicates his partners' actions. This enforces cooperation since an agent's close friends punish a defecting partner. In a world with noiseless communication, it is optimal for each agent to have close contacts to all other network members. Moreover, it is optimal to have a large network. If communication is noisy a lower number of close contacts is optimal. As the number of network members gets large norm enforcement fails.