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Path: utzoo!mnetor!seismo!rutgers!ames!ucbcad!ucbvax!TOPAZ.RUTGERS.EDU!hedrick
From: hedrick@TOPAZ.RUTGERS.EDU (Charles Hedrick)
Newsgroups: mod.protocols.tcp-ip
Subject: (none)
Message-ID: <8703200509.AA15526@topaz.rutgers.edu>
Date: Fri, 20-Mar-87 00:09:53 EST
Article-I.D.: topaz.8703200509.AA15526
Posted: Fri Mar 20 00:09:53 1987
Date-Received: Sat, 21-Mar-87 09:11:19 EST
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Organization: The ARPA Internet
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Approved: tcp-ip@sri-nic.arpa

Yes, your theory is correct.  You can make any network be treated as
local by doing
  route add <net> <yourhost> 0
(in 4.2.  In 4.3 I think there is some minor difference, like omitting the
0.)  I thought somebody had explained this before.  4.2 works like this:

   route-packet (destination-address)
	lookup destination-address as a host in host routing table
		[entries that show with "H" in flags field of netstat -r]
	if that fails, lookup network(destination-address) as a network
		in network routing table [entries without an "H"]
	if that fails, look for default route
	if that fails, say destination unreachable
	now we have a route.  
	If the metric is 1 or larger [metric is the last field in the
		"route add" comment.  You can tell from the route
		command because routes with a metric >= 1 will show
		a "G" in the flags field], this is a normal gateway
		entry.  Send packet to the host listed in the
		gateway field.
	If the metric is 0 [route add command ended in zero, or in
		some versions, the metric was omitted.  netstat -r
		does not show "G" in the flags field], this is a
		"route to the interface".  Send packet out the
		interface listed in the interface column.  If
		interface is an Ethernet, use ARP.

Note that the gateway address (gateway field in netstat -r) is
irrelevant in the case of a route to the interface.  The route add
command uses that address only to figure out which interface to
associate with the route.  So when setting up a route to an interface,
the simplest approach is to use your own address on that network.