The Intranet Concept
Three things drive TCP/IP as the protocol of choice in today's intranets: cost, speed, and extensibility. TCP/IP can be very inexpensive to implement. It can work alongside your older protocol (AppleTalk, IPX, and so on) until you migrate everything over; it works quickly and efficiently through solid and established protocols; and it can be added to on a whim via the convenience of packet switching.
The Internet is also a consideration because your company or network will now have access to this vast resource. Email is the most widely used application on the Internet today, far more even than browsing. Billions of messages are routed daily. Also, millions of Internet terminals allow people access to services that are posted to the Internet, such as sales data for a mobile workforce, an at-home workforce, or global communications.
There are many ways to connect to the Internet, but the dominant manner is dial-up and in some places DSL (Digital Subscriber Line) and Cable. Anyone with an analog modem can call an Internet service provider's modem and make a connection to the Internet. When connected, it's merely a matter of using the right resource. The dominant Data Link Layer protocol for dial-up is called Point-to-Point Protocol (PPP). An older protocol, Serial Line Interface Protocol (SLIP), allows you to make a serial connection to the Internet from a client. SLIP was later improved to provide compression of TCP/IP headers, and this led to a variation of SLIP called Compressed SLIP (CSLIP). Both SLIP and CSLIP have largely been replaced by PPP today.