Date: Thu, 27 Sep 2001 17:13:19 -0700 From: "Kevin Oberman" <oberman@es.net> To: Kutulu <kutulu@kutulu.org> Cc: Mike Porter <mupi@mknet.org>, swear@blarg.net (Gary W. Swearingen), freebsd-stable@FreeBSD.ORG Subject: Re: 127/8 continued Message-ID: <200109280013.f8S0DJk04764@ptavv.es.net> In-Reply-To: Your message of "Thu, 27 Sep 2001 18:29:26 EDT." <5.1.0.14.0.20010927182433.00a27510@127.0.0.1>
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> Date: Thu, 27 Sep 2001 18:29:26 -0400 > From: Kutulu <kutulu@kutulu.org> > Sender: owner-freebsd-stable@FreeBSD.ORG > > At 04:25 PM 09/27/2001 -0600, Mike Porter wrote: > > > >OK, then I was wrong. The broadcast is (normally) the last address in the > >subnet (.255 for a class C, .255 for my subnet ( with a .128 netmask, but I > >am in the top half. I presume that those with IP's below .129 have .127 set > >for a broadcast, with .128 being the other unusable address. I forget > >exactly what its for?) > > The two reserved addresses are the network address and the broadcast > address. The network address is all host bits zero, and the broadcast > address is all host bits 1. > > Thus, for a network of 192.168.0.0 with a netmask of 255.255.255.128, there > would be two subnets: > > 192.168.0.0 <network address> - 192.168.0.127 <broadcast address> > 192.168.0.128 <network address> - 192.168.0.255 <broadcast address> > > The exact math is a pair of pretty basic bitwise functions, which most any > networking essentials book will have in it, but that's the general > idea. But now we're really getting off the subject of freebsd-stable. :) This is a good explanation of the use of the first and last address of any CIDR block, if you are trying to get maximum use from a small space assignment (like a /29), there is really no reason to waste half of a /30. Neither the broadcast nor the network address really serve a useful purpose on /30. RFC 3021 describes a better way of addressing directly connected links so half the space is not wasted. a /31 is used for each connection allowing for 4 point to point connections from a /29. Whether FreeBSD routers can be configured to do this, I can't say, but I suspect manual route commands would do the job. I know Juniper routers support this capability. R. Kevin Oberman, Network Engineer Energy Sciences Network (ESnet) Ernest O. Lawrence Berkeley National Laboratory (Berkeley Lab) E-mail: oberman@es.net Phone: +1 510 486-8634 To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-stable" in the body of the message
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