The internet, like the telephone system which preceded it, depends for its existence on communications networks. This unit examines these networks as the means of interconnecting devices so that two-way communication is possible. Examining protocols like HTTP, TCP/IP and ATM as well as the OSI reference model, it provides and overview of the topic for learners who have significant prior knowledge of the subject. This study unit is just one of many that can be found on LearningSpace, part of OpenLearn, a collection of open educational resources from The Open University. Published in ePub 2.0.1 format, some feature such as audio, video and linked PDF are not supported by all ePub readers.