A method of controlling communication between a head-end system and a number of client systems through a network, wherein, for each receiver, only a nearest one of the multicast router systems in the network is configured to forward a copy of a message addressed to the multicast groups of which the receiver is a member directly to the receiver, includes: maintaining subscriber group information linking each registered one of the number of client systems to an associated one of a plurality of subscriber groups, and allocating registered client systems to at least one multicast group, such that all client systems associated with a particular subscriber group are in a common multicast group. Information uniquely identifying the nearest multicast router system nearest to a receiver for a first client system is obtained, and the first client system is assigned to a subscriber group including at least one further client system having the same nearest multicast router system.