using mbuffer: what am i doing wrong?

[DdB <debianlist@potentially-spam.de-bruyn.de>]

Hello list,

from my research, the abbreviated takeaway is:

> port=8000 # just an example
> filename=test.bin # created before
> 
> # Start the receiver first, like:
> mbuffer -I $port -o $filename
> 
> # Then start the sender like:
> mbuffer -i $filename -O ${receiverIP}:$port

On my LAN (all virtual, at the moment), this fails, because the sender
cannot connect to the receiver, so times out.

What left me puzzled is this:
Just starting the receiver, and then checking, if it is listening at the
$port, i found:
> sudo netstat | grep $port
to return nothing

At that point, i decided, i would appreciate some hint from people more
experienced than i am.

What am i doing wrong?
DdB