host vs hostname (Was: CID: and MID: URL schemes)

In examining the syntax of id-spec in draft-levinson-cid-01 I
considered substituting host for the existing hostname.  I.e.,
changing the present syntax from
        id-spec  := local-part "@" hostname
to
        id-spec' := local-part "@" host
where (from rfc 1738)
        host     := hostname | hostnumbmer
and
        hostnumber := digits "." digits "." digits "." digits

However, converting id-spec' into a message-ID (content-ID) becomes
problematical.  Rfc 822 requires, I think, that a message-ID containing
a host's IP address (hostnumber) have the form "[" digits "." digits "."
digits "." digits"]"

Is this reading of rfc822 correct or should id-spec be
local-part(_at_)host(_dot_)

Thanks.../Ed