Date: Wed, 11 Feb 2015 08:30:28 -0600 From: Jim Thompson <jim@netgate.com> To: Julian Elischer <julian@freebsd.org> Cc: "freebsd-net@freebsd.org" <freebsd-net@freebsd.org>, Matt Churchyard <matt.churchyard@userve.net> Subject: Re: Invalid subnet masks Message-ID: <580B0DA4-05B3-45B8-ACB9-27B9174D76A8@netgate.com> In-Reply-To: <54DB343E.7090008@freebsd.org> References: <7e069c1946454793b1c7e0be988877c4@SERVER.ad.usd-group.com> <DE405399-70FE-48A3-B550-992EDEB5C468@netapp.com> <ecc9027578ce45d7a0436e345aadc249@SERVER.ad.usd-group.com> <54DB343E.7090008@freebsd.org>
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> On Feb 11, 2015, at 4:51 AM, Julian Elischer <julian@freebsd.org> wrote: >=20 >> On 2/11/15 5:55 PM, Matt Churchyard wrote: >>=20 >> I appreciate that it might be 'valid' as a binary mask, but I'm strugglin= g to find any documentation anywhere that actually suggests that it's valid a= s a network configuration. The entire modern CIDR notation, and all the rout= ing system & hardware built around it (that shows networks in CIDR form and w= ill collapse routes) has no way of dealing with these subnets. > most can deal with it, just not optimally >>=20 >> Are there actually valid use cases for these types of network? > yes. > I've had networks that were the first and last quarter of a /24, and the m= iddle two quarters were separate nets. >=20 > Sure, it made my skin crawl, but I was in a pinch to get more machines ont= o that /26. > all four were served by the same router so only one router needed to know.= . >=20 > I have however at times though we could think about making ifconfig at giv= e a warning. > (but not an error). https://lists.freebsd.org/pipermail/freebsd-hackers/2011-April/034997.html Subject came up on -hackers in 2011 Quoting RFC-1219: "While RFC-950 allows the "ones" in the subnet mask to be non-contiguous, RFC= -950 recommends that 1) they be contiguous, and 2) that they occupy the most= significant bits of the "host" part of the internet address." Jim
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