System for managing unicast/multicast conversion for group multimedia communications in a communication network having one or more sites (11, 12, 13) and comprising at least two users (U 1, U 2), the users communicating according to a unicast mode within a site, one or more edge routers (R i), a central network organized to distribute the data streams between the different sites, characterized in that it comprises at least the following elements: ✔ At a site (11, 12, 13) and an edge router (R i), • A presence management module of the users (Ui) present on the site, • A group management module adapted for managing the membership of a user in a communication group Gi, • A control module transmitting "uplink" incoming stream and "downlink" outgoing stream conversion rules, to be applied to data of a stream entering or leaving a site and intended for one or more users belonging to a same communication group Gi, said conversion rules comprising actions for modifying the destination address of the IP header and the address of the MAC header for each user belonging at a time to a communication group and actions for duplication of the stream into one or more unicast streams depending on the number of users present on this same site and actions for conversion of the unicast stream transmitted by the user into multicast streams to transit over the core of the network to one or more users belonging to the same communication group and present on different sites from the given site. • A module adapted to apply the transmitted "uplink, downlink" conversion rules to multimedia data transmitted by a first user (U 1) before broadcasting the data modified according to the rules to one or more users (Ui) belonging to the same communication group Gi as the first user (U 1) transmitting the stream, ✔ At the central network node, Means for multicast transmission of the multimedia data streams from one site to another site. ✔ At a site (11, 12, 13) and an edge router (R i), • A presence management module of the users (Ui) present on the site, • A group management module adapted for managing the membership of a user in a communication group Gi, • A control module transmitting "uplink" incoming stream and "downlink" outgoing stream conversion rules, to be applied to data of a stream entering or leaving a site and intended for one or more users belonging to a same communication group Gi, said conversion rules comprising actions for modifying the destination address of the IP header and the address of the MAC header for each user belonging at a time to a communication group and actions for duplication of the stream into one or more unicast streams depending on the number of users present on this same site and actions for conversion of the unicast stream transmitted by the user into multicast streams to transit over the core of the network to one or more users belonging to the same communication group and present on different sites from the given site. • A module adapted to apply the transmitted "uplink, downlink" conversion rules to multimedia data transmitted by a first user (U 1) before broadcasting the data modified according to the rules to one or more users (Ui) belonging to the same communication group Gi as the first user (U 1) transmitting the stream, • A presence management module of the users (Ui) present on the site, • A group management module adapted for managing the membership of a user in a communication group Gi, • A control module transmitting "uplink" incoming stream and "downlink" outgoing stream conversion rules, to be applied to data of a stream entering or leaving a site and intended for one or more users belonging to a same communication group Gi, said conversion rules comprising actions for modifying the destination address of the IP header and the address of the MAC header for each user belonging at a time to a communication group and actions for duplication of the stream into one or more unicast streams depending on the number of users present on this same site and actions for conversion of the unicast stream transmitted by the user into multicast streams to transit over the core of the network to one or more users belonging to the same communication group and present on different sites from the given site. • A module adapted to apply the transmitted "uplink, downlink" conversion rules to multimedia data transmitted by a first user (U 1) before broadcasting the data modified according to the rules to one or more users (Ui) belonging to the same communication group Gi as the first user (U 1) transmitting the stream, ✔ At the central network node, Means for multicast transmission of the multimedia data streams from one site to another site.