The emergence of Internet-of-Things (IoT) has attracted a massive response from different stakeholders including industry, academia and research community due to the wide range of applications it provides. IoT enables devices with storage, computation and communication capabilities to connect to the Internet, thus providing various applications such as smart transportation, smart healthcare, smart industry, logistics and smart homes, to name a few. Since, IoT devices generates sensitive information, therefore, security is an important issue in IoT networks. Recently, Blockchain (BC) is proposed as a revolutionary technology which can be integrated within IoT to provide desired level of security and privacy. However, the integration of BC within IoT networks is extremely challenging. The major contribution of this study is three-fold. (1) Firstly, we proposed a tier-based architecture 'Blockchain in Internet-of-ThingS (BITS)' which can support BC within IoT networks. (2) Secondly, we highlighted various applications of BC in IoT networks, and (3) Lastly, we provided various challenges and future research directions in the realization of BC in IoT networks.