PPP (Point to Point Protocol) is a newer and better SLIP. It serves
the same purpose: sending Internet traffic over telephone (and other
serial) lines. Ask your access provider if they support it, because
otherwise you can't use it; most Internet access providers these
days support PPP rather than SLIP.

To use PPP, you need an additional program called pppd as described
in the PPP-HOWTO, available at
<http://www.tldp.org/docs.html#howto>. Make sure that you have
the version of pppd recommended in Documentation/Changes.
The PPP option enlarges your kernel by about 16 KB.

There are actually two versions of PPP: the traditional PPP for
asynchronous lines, such as regular analog phone lines, and
synchronous PPP which can be used over digital ISDN lines for
example. If you want to use PPP over phone lines or other
asynchronous serial lines, you need to say Y (or M) here and also to
the next option, "PPP support for async serial ports". For PPP over
synchronous lines, you should say Y (or M) here and to "Support
synchronous PPP", below.

If you said Y to "Version information on all symbols" above, then
you cannot compile the PPP driver into the kernel; you can then only
compile it as a module. To compile this driver as a module, choose M
here and read Documentation/networking/net-modules.txt.
The module will be called ppp_generic.

source code:

drivers/net//ppp_generic.c