In order to communicate over TCP/IP using a modem (such as through a dial-up account to an Internet service provider) or through some other serial device (such as a “null modem” serial cable between two machines), Linux provides the Point-to-Point Protocol software suite, commonly known as PPP. PPP is a protocol that takes packets sent over a network (such as TCP/IP) and converts them to a format that can be easily sent over a modem or serial wire. Chances are, if you have an Internet account with an ISP, the ISP’s server uses PPP to communicate with dial-up accounts. By configuring PPP under Linux, you can directly connect to your ISP account in this way.
SLIP is an earlier protocol that has the same basic features as PPP. However, it lacks certain important qualities, such as the ability to negotiate IP addresses and packet sizes. These days SLIP has more or less been supplanted entirely by PPP; however, some older ISPs may still use SLIP rather than PPP. If this is the case for you, we refer you to other sources of information, such as the Linux Network Administrator’s Guide (O’Reilly).
In this section, we will cover configuration of a PPP client — that is, a system that will connect to an ISP (or other PPP server) in order to communicate with the Internet. Setting up a Linux machine as a PPP server is also possible but is somewhat more involved; this is covered in the Linux Network Administrator’s Guide (O’Reilly).