Ensure you have a laplink cable (often yellow coloured) connected to the interfaces of the two hosts to be linked. Host 1 has IP 192.168.42.42, – host 2 has ip 192.168.42.254.
Host 1
# ifconfig lp0 192.168.42.42 192.168.42.254
lp0: flags=8851<UP,POINTOPOINT,RUNNING,SIMPLEX,MULTICAST> mtu 1500
inet 192.168.42.42 –> 192.168.42.254 netmask 0xffffff00
Host 2
# ifconfig lp0 192.168.42.254 192.168.42.42
lp0: flags=8851<UP,POINTOPOINT,RUNNING,SIMPLEX,MULTICAST> mtu 1500
inet 192.168.42.254 –> 192.168.42.42 netmask 0xffffff00
Now, as both interfaces are up you can use them as if they were real ethernet interfaces. The bandwidth is theoretically about 160kb/s. High transfers result in high work load due to exhaustive use of IRQs. On slower, think of 80486 and Pentium I systems, the load can increase significantly and the bandwidth can decrease vice versa.
I.e. in the latter case my AMD 486 DX4/133 PCM4823l Embedded Router processes about 30kb/s at a cpu load of 100%, with overall system load of about 1.2. My PIII-450 Katmai system processes about 80kb/s at a cpu load of ~ 60%. My AMD 2400+ Thunder-Whatever system processess 90kb/s with a cpu load of ~ 5%.