From: mea@sparta.com (Mike Anderson)
Newsgroups: comp.os.vxworks
Subject: re: Ethernets and backplane networks
Date: Tue, 17 Nov 92 18:59:49 EST
Organization: Lawrence Berkeley Laboratory, Berkeley CA
NNTP-Posting-Host: 128.3.112.16
Originator: daemon@vxw.ee.lbl.gov

Greetings!

Bill Frewing writes:
>  
> Mike Anderson replied to my questions about using up internet network 
> numbers (with 'private' ethernets for VxXorks crates and backplane 
> networks) with a fairly comprehensive posting about subnets. I have
> a further question on that subject: do the subnets have to be physically
> connected ?  Ie. could the subnets be arranged as follows where the
> subnets have netmask 0xffffffc0:
> 
>                     Internet G/W             Main Ethernet (not subnetted)
> Internet ----------[192.9.200.1]----------------------------------------------
> (sees only                       |                |
>  net 192.9.200.0)                |                |
>                          ---------------   ---------------
> 	Suns w/ 2 Enets  [ 192.9.200.2 ]   [ 192.9.200.3 ] <- main Enet
> 			 [ 192.9.201.1 ]   [ 192.9.201.64] <- subnetted Enet
>                          ---------------   ---------------
>                                  |                |
>                                  |                |
>            (00 sub-network)  ---------            |
>                                               ----------  (01 sub-network)
> 
> I would guess that the problem may be that each Sun could only have one
> netmask, and for the above configuration it would need a different one for
> the main network (192.9.200) and the sub-netted network (192.9.201).
> 

  By virtue of the fact that you're using a class C network, the 192.9.201.xxx networks
are completely separate *networks* from the 192.9.200.xxx network not subnets.  However,
if you set the network mask to 0xffffffc0 then that means that you would now have 4
subnetworks on the 192.9.201 net and 4 subnets on the 192.9.200 net.  This is possible
to handle via clever uses of routing table entries, but I would exercise caution about
trying to use *different* subnet masks on the two Enet interfaces.  I have had problems
doing this because the routing table takes the netmask into account when allowing you
to perform the "route add net" calls.  Also, broadcasts can be a pain because, by design,
gateways do not forward broadcast packets (just image what would happen to the Internet
if every ARP broadcast got forwarded from a LAN out into the world! :-).  Therefore, such
a set up would work provided that the 00 subnet (192.9.201.xx) did not expect to broadcast
packets to the 01 subnet.  You might also have to use a "default" route (0.0.0.0) 
to the Sun with 2 Enets for packets to be routed properly like so:

-> hostAdd "default", "0.0.0.0"
-> routeAdd "default", "192.9.201.1"   # this handles the 00 subnet and do likewise on 01

Hope this helps,


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Newsgroups: comp.os.vxworks