In the previous chapter, we saw how IP packets carry TCP segments or UDP datagrams across networks. Now it is time to look at what happens in the top layer of a TCP/IP-based network, the application layer. This chapter starts with an explanation of client–server technology, which underlies most Internet activities. We examine the following applications in turn: the domain name system (DNS), the World Wide Web, remote access, file transfer, E-mail, the delivery of streamed content over the Internet and voice over IP (VoIP). We discuss the main protocols for each of these applications. The chapter ends with brief descriptions of peer-to-peer (P2P) file sharing, instant messaging (IM) and microblogging.