Parallel LapLink Cable
DB25 Parallel Data Link Cable
D-Sub 25-pin Lap-Link Male to Male
Null Printer Cable
CB-LINK-25M-25M-xx

Parallel Laplink cable (also known as null-printer cable) can be used to connect two PCs and allows them to communicate via the application program LapLink™.

The connection is achieved via the parallel ports on the two computers. No networking hardware such as a network interface card or a modem are required. The cable is named after the Laplink software package, which was designed to make such a connection possible (many years before it became a Windows term and feature). It can also be used by the InterLink program in MS-DOS 6.
 
A Laplink cable can be seen as a parallel equivalent to a serial null modem cable. Because of the higher bandwidth of the parallel port versus the serial port, a Laplink cable is able to shift data more quickly at the same clock speed, which was important at the time it was developed, since clock speeds then were relatively slow.

This is still the reason it is used (rather than a RS-232 port) as a means to install GNU/Linux to machines without a CD-ROM/DVD station or Ethernet port built-in. And can also be used as an (additional) failover link between ha-cluster nodes.