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From: tkevans@oss670.UUCP (Tim Evans)
Newsgroups: comp.protocols.tcp-ip
Subject: Curious Behavior of 4.2BSD TCP/IP
Keywords: subnetting
Message-ID: <627@oss670.UUCP>
Date: 21 Feb 91 19:45:29 GMT
Organization: Social Security Admin., Baltimore/Washington
Lines: 62

I have a VAX 11/750, with 4.2BSD (NOTE:  4.2, not 4.3), which I'm
trying to place onto my network and am having curious results.  (Before
describing them, I have to say that I cannot upgrade to 4.BSD, so
must deal with the situation as is.)

Our network formerly consisted of 6 separate networks, which have only
recently been bridged with hardware bridges into a single spanning
tree network.  Prior to the bridging, I'd implemented subnetting in
our class B address space, with a netmask of 255.255.255.0.  Each 
of the 6 networks was a subnet.  Now, I still maintain the fiction of
having 6 subnets, even though they're all the same cable.  (I'd really
rather not have to go back to all machines and rip out the subnetting
stuff.)

I set up /etc/networks showing the 6 "subnets."  Here's what it looks
like:

loopback	127
oss-lan		137.200
lan1		137.200.1 localnet	# VAX is on this subnet
lan2		137.200.2
lan3		137.200.3
lan4		137.200.4 
lan5		137.200.5
lan6		137.200.6

In addition, the VAX's rc.local installs routes to each of the 5 "subnets"
outside its own, as shown below:

route add lan2 `hostname` 0
route add lan3 `hostname` 0
route add lan4 `hostname` 0
route add lan5 `hostname` 0
route add lan6 `hostname` 0

The VAX, which is on "subnet" 1 can see and access all machines on
"subnets" 1, 2, and 3, but not on 4, 5, and 6.  Similarly, machines
on "subnets" 1-3 can telnet/ftp to the VAX, but those on 4-6 cannot.

The next curious thing is that even after installing the routes to
the "subnets," 'netstat -r' doesn't report them.  All that's reported
is the route to "osslan"  (see networks file above).

More interestingly, if I reassign the VAX's IP address to one in 
another "subnet" (say, "subnet" 5, one that's *not* reachable) and
modify the /etc/networks and installed routes accordingly, I have the
same results as before!  That is, even with an IP address in "subnet" 5,
the VAX can see *only* "subnets" 1-3, and not 4-6.

Now, I could understand it if the VAX couldn't see *anything* outside its
own "subnet"  (4.2BSD doesn't know about subnets, right?), but the fact
that it can see some but not others is really confusing.  And, that it
can see/not see the same subnets, regardless of its own "subnet,"
makes it even more curious.

Comments?  Suggestions?  (That is, other than upgrading the BSD.)
Thanks.
-- 
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