Path: utzoo!attcan!uunet!lll-winken!uwm.edu!rpi!zaphod.mps.ohio-state.edu!usc!ucsd!ucbvax!VENERA.ISI.EDU!braden
From: braden@VENERA.ISI.EDU
Newsgroups: comp.protocols.tcp-ip
Subject: Re:  Hosts whose IP numbers end in 0...
Message-ID: <9008311641.AA02779@braden.isi.edu>
Date: 31 Aug 90 16:41:21 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 52


	From @jessica.stanford.edu:postel@venera.isi.edu Wed Aug 29 18:16:23 1990
	Date: Wed, 29 Aug 90 18:12:35 PDT
	From: postel@ISI.EDU
	To: ietf-hosts@nnsc.nsf.net, ietf-rreq@jessica.stanford.edu,
	        tcp-ip@nic.ddn.mil
	Subject: Hosts whose IP numbers end in 0...
	Cc: postel@ISI.EDU

	Hi.

	I believe the following analysis by David L Stevens is correct.  Some
	discussion like this should be added to all requirements documents and
	other descriptions of address filtering and/or broadcast addresses.

	--jon.

	~~~~~~~~~         ----- Begin Included Message -----        ~~~~~~~~~~
	Date: 29 Aug 90 16:31:45 GMT
	From: usc!wuarchive!zaphod.mps.ohio-state.edu!samsung!noose.ecn.purdue.edu!mentor.cc.purdue.edu!dls@ucsd.edu  (David L Stevens)
	Subject: Re: Hosts whose IP numbers end in 0........
	Sender: tcp-ip-relay@nic.ddn.mil
	To: tcp-ip@nic.ddn.mil


		A point of clarification, since several people have sent me e-mail
	on the topic...

		When I say "shouldn't" regarding filtering, I mean in the RFC-speak
	sense of "it's a bad idea." I don't mean it's contrary to any RFC, so you
	don't need to point out to me that some say it's fine for gateways to filter
	broadcast packets.
		Beyond that, there is the fact that it is impossible to do it right
	in the case we were talking about, and THAT is just plain wrong. A gateway
	that isn't directly attached or administered by the same people as an
	arbitrary IP address CANNOT answer the question "is this a broadcast?" so
	it doesn't matter whether an RFC says you can or not; in practice, you can't.

Jon and David,

Sorry, I think there is a confusion here.  Filtering out directed
broadcasts is neither a "bad idea" nor "impossible"!!  This means: those
broadcasts that the given gateway can legally and possibly detect.  Of
course, it does NOT mean breaking the rules about invisibility of
subnets outside a subnetted network, and it does NOT mean making
illegal assumptions about byte boundaries within subfields of the IP
address.

Jon thinks David and I are having an agreement, but I am unclear
about that.

Bob Braden