gdbserver host:port question

Hi ,

as far as I know gdbserver can be launched with above command ,
specifying where the server should listen for incoming traffic.

however the part that confuses me is the host 'part', specifically

does host mean the IP address from which the client (gdb-multiarch)
will send its command, or the IP address relative to the host the
server is running on?

And finally will the syntax gdbserver :port be supported without
specifying the host?

Thanks