Internetworking: The Internet, TCP/IP

The aim of internetworking is to connect two or even more networks with the result that they look like one single, virtual network to the user.
This network is called an internet; the networks it consists of are referred to as subnetworks. To reach this goal different interconnection devices are needed.

Internet with a capital "I" refers to a worldwide internet built of large national backbone networks. So, the Internet is a connection of WANs.
The Internet provides the following applications in an international scope:

• Simple Mail Transfer Protocol (SMTP): allows to send electronic mail (EMail) to thousands of other computers all over the world
• File Transfer Protocol (FTP): allows sending and receiving files from one system to the other under user command
• Telnet: provides the capability to log on to a remote computer system. The user then is able to act as if directly connected to that system.
• Network File System (NFS): enables the user to work with a remote filesystem, as if it were on the local computer (transparent)

Guide to the Internet

Unfortunately, the Internet does not work with protocols according to the OSI model. Instead it uses its own suite of protocols called TCP/IP (Transmission Control Protocol/ Internet Protocol), named after two involved protocols. A comparsion between OSI model and TCP/IP protocol suite shows the differences:

The protocols of the Network access layer are not specified by TCP/IP. A protocol appropriate for a particular network is used. The Internet layer protocol is IP (Internet Protocol); transport is handled by TCP (Transmission Control Protocol). For the Application layer the above mentioned SMTP, FTP, Telnet etc. are laid down.

There is also a more comprehensive Introduction to TCP/IP available.