SUMMARY: Two IP interfaces in the same subnet: possible ?

From: Thomas Leitner <tom_at_radar.tu-graz.ac.at>
Date: Wed, 18 Aug 1999 16:29:43 +0200 (MET DST)

Hi everybody,

I got 0 (in words: zero) replies to my posting. Maybe because it was too
trivial, a little bit off-topic, or I didn't explain thoroughly enough
what I wanted. In any case: It works with DU 4.0D and no patch-kit. I've
just put the second ethernet interface 'ln1' in the same subnet. The
rt-device keeps its IP address. I've added a static host route
like this in /etc/routes

-host rt-device -interface ip-of-ln1-interface

Now all used ip addresses are in the same subnet (mask=255.255.255.244)
and it still works. Apparently the static host route really has priority
over the default route and DU chooses the right interface for the
destination. "netstat -r" now has:

default 192.168.159.33 UGS 10 1707702 ln0
localhost localhost UH 1 4 lo0
192.168.159.32 192.168.159.35 U 7 1732932 ln0
rt-device 192.168.159.36 UHS 3 544644 ln1

BTW: I was unable to dynamically change the IP address of any IP
interface! I've tried:

- changing /etc/rc.config followed by /sbin/init.d/inet stop/start
- manually "ifconfig ln0 down; ifconfig ln0 192.168.159.35; ifconfig ln0
  up"

Tom

P.S: Here's my original posting:

------------------------------------------------------------------------

Hi,

I'd need a quick answer to this: Tomorrow I'll put in a second
ethernet card into a DEC3000/600 in order to have an isolated LAN
to a real-time device. Currently all nodes are in the same subnet
like this:
                 ln0
        DEC 3000 ----!
        RT-DEVICE ---!
WAN---- ROUTER ------!
           1 LAN SEGMENT, 1 IP Subnet

What I have in mind is this:

                  ln1 ln0
              !------ DEC 3000 ----!
   RT-DEVICE -! ROUTER ------!
           LAN SEG 2 LAN SEG 1
                    1 IP Subnet

Currently the DEC 3000 has the default route set to the Router.

Question: Can I just keep the IP address of the RT-DEVICE, assign a free
IP address in the same subnet to ln1 and define a static host route to the
RT-DEVICE ? Or differently put: Can I have two IP interfaces in the same
IP Subnet? Will the static host route have priority over the default route?

Thanks // Tom
-- 
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T o m   L e i t n e r                       Dept. of Communications
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Received on Wed Aug 18 1999 - 14:33:57 NZST

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