This technical report introduces the concepts, problems and a possible solution for ROS multi-master systems, that is, systems build from two or more ROS networks, each with its own roscore node. In general this environment would correspond to multi-robot systems, either mobile platforms or manipulators. The ROS framework already provides a solution for such systems, multimaster_fkie, which is presented and briefly described in thisThe ROS framework already provides a solution for such systems, multimaster_fkie, which is presented and briefly described in this technical report, together with the network setup necessary to make it work properly. Two different configurations are discussed in this technical report, simple ROS networks with a single computer each, and more complex ROS networks with two or more computers each. In both cases, real examples are provided using robots available at IRI.