The Internet of Things (IoT) is gaining increasing attention. The overall aim is to interconnect the physical with the digital world. Therefore, the physical world needs to be measured and translated into processible data. Further, data has to be translated into commands to be executed by actuators. Due to the growing awareness of IoT, the amount of offered IoT platforms rises as well. The heterogeneity of IoT platforms is the consequence of multiple different standards and approaches. This leads to problems of comprehension, which can occur during the design up to the selection of an appropriate solution. We tackle these issues by introducing an IoT reference architecture based on several state-of-the-art IoT platforms. Furthermore, the reference architecture is compared to three open-source and one proprietary IoT platform. The comparison shows that the reference architecture provides a uniform basis to understand, compare, and evaluate different IoT solutions. The considered state-of-the-art IoT platforms are OpenMTC, FIWARE, Site-Where, and Amazon Web Services IoT.