The invention concerns a multiplexing unit, a system and a method for communication in a computer network between a plurality of client machines supporting client programmes and one or several servers supporting application programmes, although said client and application programmes may be incompatible. The invention uses input and output management modules assigned to the client machines and to the servers. It performs conversion operations and routing operations, which are carried out so as to optimise the operation of the servers. It ensures communication between applications having incompatible formats or transfer protocols and this by means of a flexible technique with characteristics of modularity and upgradability. The invention is applicable in particular to computerised reservation systems for example in the field of travels and transport.