These allow three UTP or ISDN cables to be coupled. They will allow two computers to share one connection point, but with only one computer in operation at a time (i.e. this is a simple double adapter, NOT an Ethernet hub or switch).
Internal wiring is as follows:
Socket 1, Pin 1 connects to socket 2, pin 1 and socket 3, pin 1
Socket 1, Pin 2 connects to socket 2, pin 2 and socket 3, pin 2
Socket 1, Pin 3 connects to socket 2, pin 3 and socket 3, pin 3
All 8 lines are thus connected.