Hi,

I guess you are reading the book about device drivers from O'Reilly.
As far as I remember snull is an example network driver that creates 2 interfaces. It works like loopback split in 2 devices.
Traffic send through one of the interfaces will be received at the other.

The book was written for kernel 2.4. I don't know if the code still works without modifications.

Hope it helps
Basti