What Is SLIP/PPP?

In the discussion of various means of internet access (previous page), we mentioned that a direct connection to the internet represents the ultimate mode of access. This is, however, often too expensive for the individual user. The alternative, using host dial-up access to a shell account, may be less than satisfactory for some users (eg., you cannot use GUI based web browsers, ftp directly into your own PC, etc).

SLIP/PPP Overview

A compromise between a direct internet connection and host dial-up access is to use SLIP (Serial Line Internet Protocol) or PPP (Point to Point Protocol). From now, we'll just say SLIP/PPP to refer to both SLIP and/or PPP - they are similar in many ways.

SLIP/PPP provides the ability to transport TCP/IP traffic over serial lines, such as dial-up telephone lines, between two computers. Both computers run some sort of TCP/IP based network software. This allows a home user to get direct internet access from his own PC with just a simple modem and a telephone line. For many users, this is an exciting way to get direct internet connectivity at a low cost. With SLIP/PPP, you can run your favourite GUI based web browser, ftp client, etc - right from your own PC.

SLIP/PPP is really a form of direct internet connection in the sense that:

Your computer has a communications link to the internet, even if it is via a service provider;
Your computer has the networking software that can speak TCP/IP with other computers on the internet;
Your computer has an identifying address (IP address) at which it can be contacted by other computers on the internet.

SLIP/PPP vs Host Dial-Up Access

Both SLIP and normal dial-up access involve dialing into a remote computer system (which is directly on the internet) and logging in. Some people may have difficulty understanding the difference. The key distinction is that with SLIP, your own PC is communicating using native TCP/IP with other computers on the internet while with normal host dial-up, your PC simply acts as a dumb terminal to the remote computer which then communicates with other computers on the internet using native TCP/IP.

To further elaborate:

When you run a ftp client with host dial-up, the files you receive are stored on the remote computer (which you later download using Zmodem or a similar file transfer protocol. With SLIP/PPP, however, you run the ftp client on your PC and the files are received directly on your own PC.
With SLIP/PPP, you can run a GUI based web browser on your own PC. With host dial-up access, you must run the web browser (a text based program like Lynx) on the remote computer.
It is possible (although there may be complications in practice), with SLIP/PPP, to run server applications on your PC such as a FTP server so that users elsewhere on the internet can send or retrieve files from you. There is no equivalent for dial-up host access.

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