PLIP stands for Parallel Line IP and is a cheap way to network when you want to connect only two machines. It uses a parallel port and a special cable, achieving speeds of 10kBps to 20kBps.
PLIP was originally developed by Crynwr, Inc. Its design is rather ingenuous (or, if you prefer, hackish): for a long time, the parallel ports on PCs used to be only uni-directional printer ports; that is, the eight data lines could only be used to send from the PC to the peripheral device, but not the other way round. PLIP works around this by using the port's five status line for input, which limits it to transferring all data as nibbles (half bytes) only. This mode of operation is called mode zero PLIP. Today, these uni-directional ports don't seem to be used much anymore. Therefore, there is also a PLIP extension called mode 1 that uses the full 8 bit interface.
Currently, only supports mode 0. Unlike earlier versions of the PLIP code, it now attempts to be compatible with the PLIP implementations from Crynwr, as well as the PLIP driver in NCSA telnet. To connect two machines using PLIP, you need a special cable sold at some shops as ``Null Printer'' or ``Turbo Laplink'' cable. You can, however, make one yourself fairly easily. Appendix shows you how.
The PLIP driver for is the work of almost countless persons. It is currently maintained by Niibe Yutaka. If compiled into the kernel, it sets up a network interface for each of the possible printer ports, with plip0 corresponding to parallel port lp0, plip1 corresponding to lp1, etc. The mapping of interface to ports is currently this:
If you have configured your printer port in a different way, you have to change these values in drivers/net/Space.c in the kernel source, and build a new kernel.
This mapping does not mean, however, that you cannot use these parallel ports as usual. They are accessed by the PLIP driver only when the corresponding interface is configured up.