From owner-freebsd-isp Tue Oct 8 09:09:14 1996 Return-Path: owner-isp Received: (from root@localhost) by freefall.freebsd.org (8.7.5/8.7.3) id JAA04164 for isp-outgoing; Tue, 8 Oct 1996 09:09:14 -0700 (PDT) Received: from saguaro.flyingfox.com (saguaro.flyingfox.com [204.188.109.253]) by freefall.freebsd.org (8.7.5/8.7.3) with SMTP id JAA04157 for ; Tue, 8 Oct 1996 09:09:11 -0700 (PDT) Received: (from jas@localhost) by saguaro.flyingfox.com (8.6.12/8.6.10) id JAA24732; Tue, 8 Oct 1996 09:06:24 -0700 Date: Tue, 8 Oct 1996 09:06:24 -0700 From: Jim Shankland Message-Id: <199610081606.JAA24732@saguaro.flyingfox.com> To: fbsd-isp@launchpad.win.net, freebsd-isp@FreeBSD.org Subject: Re: Subnetting Sender: owner-isp@FreeBSD.org X-Loop: FreeBSD.org Precedence: bulk > I have a question about subnetting. Why is it a no-no to use all > zero (or all 1 for that matter) subnet numbers? Everything says, > "Don't do it," but nothing says why, beyond the fact that it's > non-standard. In the end everything points at RFC 950, which just > says, "It's not recommended," again without giving a reason.... > Is there some specific *technical* reason for not doing it? It means you can't tell the difference between a subnet-directed broadcast and a network broadcast. Subnet-directed broadcast: Net value: x Subnet value: y Host value: all 1's (or all 0's if brain-damage) All subnets broadcast: Net value: x Subnet value: all 1's (or all 0's if brain-damage) Host value: all 1's (or all 0's if brain-damage) If you allow a subnet value of all 0's (or all 1's), then you can't tell these two cases apart. Whether this is enough reason to avoid these subnets is debatable. Jim Shankland Flying Fox Computer Systems, Inc.