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From: simon@sirius.ucs.adelaide.edu.au (Simon Hackett)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: An INTERESTING problem
Message-ID: <2243@sirius.ucs.adelaide.edu.au>
Date: 11 Jan 91 01:01:04 GMT
References: <9101100248.AA01030@desktalk.desktalk.com>
Sender: news@ucs.adelaide.edu.au
Reply-To: simon@sirius.ucs.adelaide.edu.au (Simon Hackett)
Organization: Information Technology Division, The University of Adelaide, AUSTRALIA
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Nntp-Posting-Host: sirius.ucs.adelaide.edu.au


You don't have to waste a subnet number on each sl/ip link. You can configure each end of the link using the primary IP address of each system as the addresses configured into the sl/ip interface at each end of the sl/ip link.

i.e. for a link thus:

   ----(ether)----[HOST A]--(sl/ip)--[HOST B]---(ether 2)---

If HOST A and HOST B have primary IP addresses for each of their respective ethernets, set up the sl/ip interface on HOST A so the ip address of the local side of the sl/ip link is the same as host A's ethernet address, and set the IP address of the remote end of the sl/ip link to the ethernet IP address of HOST B. Vice versa at the other end.

What happens under these conditions? Well, it might look illegal, but although it skates on the edge, it works: For routing out of HOST A to HOST B, all the matters is the remote sl/ip interface address - so packets get sent correctly to host B. They have a source IP address of [HOST A's ethernet IP number], but this is perfectly valid for them.

Packets on the way back work the same way - since they are tagged as having originated from HOST A's ether address, they get routed to HOST A via the SL/IP link. 

Hopefully this makes sense. If you imagine you're an IP router and go through the steps involved in routing packets each way, you'll see that it works out right.

Simon Hackett
Adelaide Uni