The application layer includes many protocols. The transport layer includes fewer, most notably, TCP and UDP. The TCP/IP network layer includes a small number of protocols, but only one major protocol: the Internet Protocol (IP). In fact, the name TCP/IP is simply the names of the two most common protocols (TCP and IP) separated by a /.
IP provides several features, most importantly, addressing and routing. This section begins by comparing IP’s addressing and routing with another commonly known system that uses addressing and routing: the postal service. Following that, this section introduces IP addressing and routing. (More details follow in Chapter 4, “Fundamentals of IPv4 Addressing and Routing.”)