Say Y if you intend to use SLIP or CSLIP (compressed SLIP) to
connect to your Internet service provider or to connect to some
other local Unix box or if you want to configure your Linux box as a
Slip/CSlip server for other people to dial in. SLIP (Serial Line
Internet Protocol) is a protocol used to send Internet traffic over
serial connections such as telephone lines or null modem cables;
nowadays, the protocol PPP is more commonly used for this same
purpose.

Normally, your access provider has to support SLIP in order for you
to be able to use it, but there is now a SLIP emulator called SLiRP
around (available from
<ftp://ibiblio.org/pub/Linux/system/network/serial/>) which
allows you to use SLIP over a regular dial up shell connection. If
you plan to use SLiRP, make sure to say Y to CSLIP, below. The
NET-3-HOWTO, available from
<http://www.tldp.org/docs.html#howto>, explains how to
configure SLIP. Note that you don't need this option if you just
want to run term (term is a program which gives you almost full
Internet connectivity if you have a regular dial up shell account on
some Internet connected Unix computer. Read
<http://www.bart.nl/~patrickr/term-howto/Term-HOWTO.html>). SLIP
support will enlarge your kernel by about 4 KB. If unsure, say N.

To compile this driver as a module, choose M here and read
Documentation/networking/net-modules.txt. The module will be
called slip.

source code:

drivers/net//slip.c