Abstract: A network node preprocesses Closed User Group (CUG) member rules and network configuration information available to the node during a configuration phase of the network preceding an operational phase in which calls are placed among the users. The preprocessing stores the information about the CUG membership of network users into a preprocessed CUG information table directly associated with user addresses. CUG membership of some attached users is ambiguous at configuration time if there exists a CUG member rule that is more specific than a prefix configured on the node with which the addresses of such users begin. For such users, CUG membership is determined at call time. The node determines at configuration time whether the CUG membership information concerning users whose addresses begin with each prefix configured on the node is ambiguous, and for those prefixes for which user CUG membership is ambiguous an associated indicator is set.