Re: broadcast to internet?
From: Barry Margolin (barmar_at_alum.mit.edu)
Date: 10/14/04
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Date: Wed, 13 Oct 2004 20:46:15 -0400
In article <slrncmqss2.64o.eirik@kain.mi.uib.no>,
Eirik Seim <eirik@mi.uib.no> wrote:
> On 13 Oct 2004 11:05:03 -0500, briggs@encompasserve.org wrote:
> > In article <slrncmqeif.3kq.eirik@kain.mi.uib.no>, Eirik Seim
> > <eirik@mi.uib.no> writes:
> > > On 13 Oct 2004 06:06:37 -0700, Bernd M?ller wrote:
> > >> Tauno Voipio <tauno.voipio@iki.fi.NOSPAM.invalid> wrote in message
> > >> news:<tgdad.229$xb5.195@read3.inet.fi>...
> > >
> > > [snip]
> > >
> > >> > The address 255.255.255.255 is the local link broadcast address.
> > >> >
> > >> you mean: 255.255.255.1 !
> > >
> > > Actually, I don't think he do. But "255.255.255.255" is bougus,
> > > and says really nothing of IP layer broadcasts. It's simply how
> > > applications interpret the local link layer broadcast address,
> > > which is ff:ff:ff:ff:ff:ff.
> >
> > No. 255.255.255.255 is a real, honest to goodness IP address. It
> > fits into 32 bits in the destination address field in the IP header
> > just like an IP address should. And that's where it is placed.
> >
> > IP applications should never see the link layer source address
> > or the link layer destination address, even assuming that
> > there is such a thing as a link layer source address or link
> > layer destination address on the network in question. There
> > may not be. Don't let your parochial "everything is on Ethernet"
> > point of view mislead you.
>
> I'm not sure who sold you my point of views, but apparently
> you should claim a refund. My point was to illustrate that
> several applications (and operating systems, whatever) translates
> the link layer broadcast address into "IP address" 255.255.255.255
This is *not* what happens. Briggs is correct that applications never
see the link layer address. The packet doesn't get to the application
unless the IP layer has already accepted the packet, which happens only
if the destination IP address is one of the host's unicast addresses, a
multicast address that the host has joined, or a broadcast address.
The translation you're talking about happens in the other direction. If
an application sends a packet to IP 255.255.255.255, the IP stack should
transmit it as a link layer broadcast. Also, RFC 1122 says that a host
should (but is not required to) ignore a packet received via a link
layer broadcast if the IP destination is a unicast address.
> because it's a packet, and for whatever reason their way of
> representing this does not fit for a packet (or frame, actually)
> without any IP information. I never said anything about Ethernet.
You said ff:ff:ff:ff:ff:ff, which looks like an Ethernet address.
Actually, it's an 802.x address -- a number of LAN technologies
(Ethernet, FDDI, Token Ring, etc.) share the same link layer addressing
scheme. But it's certainly not the only link layer addressing scheme
there is; for instance, ATM and Frame Relay don't use that address
format.
>
> The rest is of course valid with regard to IP, but I'm pretty
> sure IP has nothing to do with this. As far as I can see, this is
> normal behaviour of a protocol considered non-routable. Perhaps
> I should have said "bougus in this context".
What protocol and context are you talking about? The OP mentioned a
firewall log that showed IP packets destined to 255.255.255.255. Since
this is IP, it's a routable protocol.
-- Barry Margolin, barmar@alum.mit.edu Arlington, MA *** PLEASE post questions in newsgroups, not directly to me ***
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