difference btw splnet & splimp

[Larry Allen]

Not quite.  The network input code called from device interrupt handlers
requests a software interrupt at the net ipl.  Raising priority to splnet()
blocks the software interrupt from occurring until the ipl is lowered
again.  This is important in synchronizing access to the network data
structures (like the queues of mbufs on the send and receive sides of
sockets).

There is some relatively tricky synchronization going on here among the
device interrupts, network software interrupt, and process-level code.
I would recommend not messing with it unless you completely understand it.
					-Larry Allen