Date: Thu, 09 May 2002 11:04:44 -0500 From: Steve Fettig <lists@stevenfettig.com> To: Ceri Davies <setantae@submonkey.net>, Christopher Schulte <schulte+freebsd@nospam.schulte.org> Cc: f-q <freebsd-questions@freebsd.org> Subject: Re: setting up IP aliases in rc.conf Message-ID: <B900084C.83FC%lists@stevenfettig.com> In-Reply-To: <20020509075848.GA5768@submonkey.net>
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On 5/9/2002 2:58, "Ceri Davies" <setantae@submonkey.net> wrote: > On Wed, May 08, 2002 at 05:13:36PM -0500, Christopher Schulte wrote: <snip> >> A 15 second search produces: >> >> http://groups.google.com/groups?selm=fa.ijraokv.1riqagp%40ifi.uio.no&output=g >> plain > > "Otherwise it doesn't work" isn't really the answer that Steve was looking > for, I'd hazard. > > A better answer is here : > > http://marc.theaimsgroup.com/?l=freebsd-questions&m=100818653932698&w=2 > > Ceri Ceri, Actually, you are right - the answer brought me a bit further, but still left me with the question "why?" I tried to post last night - something to the effect of "why" - but apparently it was either bounced or didn't make it out of my mail manager. Anyway, thanks for the post. This makes things a bit clearer for me. I think I understand it, but wanted to see if others could concur: The subnet mask is *really* for the machine sending out the data, so it knows which interface, network, and gateway is to be used. As far as an outside machine is concerned, it is oblivious to the subnet mask and only knows the IP - so when it sends data to that IP/interface, it isn't necessarily paying attention to the subnet mask anyway. The machine with multiple IP's, on the other hand, needs that information to again, know the route to use when sending data outbound. As for how the machine communicates with itself between aliases, it seems that the comment Crist J Clark makes about the address with a mask of 0xffffffff means (to me) that the machine is forced to use the IP that actually has a broadcast network associated with it. I was reading up on IP datagram creation and found this quote to be somewhat helpful: "Imagine that a gateway is connected to three different IP networks. When the gateway receives an IP datagram, it looks at the network address portion of the datagram's destination address and then looks to see if it knows how to reach that network...If the datagram is not destined for one of the networks to which the router is connected, the packet is forwarded to another gateway, or discarded. This same process is performed by end-stations. When an end station creates an IP datagram, it takes the destination address of the datagram, compares it to its own address, and then uses its subnet mask to see if the network addresses match. The datagram is either forwarded on to the destination or is sent to the gateway. The subnet mask is clearly important and will cause problems if not set correctly." (45 Martin)** This quote doesn't discuss directly what started this whole conversation, but the issue of matching network addresses should guide one in the right direction. In a way, an address with a mask of 255.255.255.255 is a network in itself because all of the octets of the IP are read to *be* the network. Boy, talk about difficult to put into words. I hope I have made some sense of this. If so, maybe I can actually come up with a short primer on *why* this works the way it does... Thanks, Steve **"Understanding the Network: A Practical Guide to Networking," Michael J. Martin, Copyright 2000 New Riders Publishing To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-questions" in the body of the message
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