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Date:      Thu, 12 Sep 1996 02:21:59 +0200 (MET DST)
From:      Jos Vissers <>
To: (Bill Fenner)
Subject:   Re: Why does arp not work when ip-alias installed
Message-ID:  <>
In-Reply-To: <> from "Bill Fenner" at Sep 11, 96 09:09:02 am

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Bill Fenner wrote:
> >We have a network with several Livingston portmaster and a
> >cisco router as gateway. Most of the dial-up ip addresses are on
> >the same class c network as the servers and therefore arp -a
> >should give the ethernet address of the portmaster for an
> >address that is in use.
> >
> >It does on machines without an ip-alias.
> >It doesn't on machines with ip-aliases.
> Are machines with IP aliases still able to contact these hosts?

No, they cannot be reached:
PING ( 56 data bytes
ping: sendto: Host is down
ping: wrote 64 chars, ret=-1

This is my own machine at home, I could log in to another
server and then telnet to this one so it should definitely
have been reacheable. From the dial-up host the server just
doesn't respond. It apparently knows how to get packets to
the server, but the server doesn't know how to get them back.

> What does "netstat -nr" show on the machines with aliases?

Without routed it shows nothing abnormal until you ping one of
the dial-up hosts that are connected at the time.
It wil not show in the routing table and arp -a shows: ( at (incomplete)

If I run routed it shows the portmasters ip as gateway which is correct:
usquebauch         pm2.nmg            UGH         0        3       ed0

but arp -a doesn't show any of the dial-up hosts.

It did some further testing and it only appears to happen
when I add an ip-alias in another class-c network.
I used "ifconfig alias netmask 0xffffff00" and
the machine's regular ip address is

When I remove the alias again it refuses to start working.

The problem doesn't occurr when I define aliases in the same
class-c with - of course - a netmask of 0xffffffff.

Is this my error? Should I use 2 different ethernet cards
when using different class-c addresses on one machine?
Another of our servers with 2 cards doesn't have this problem.

Thanks for the help, Jos

   Jos Vissers, System administrator Telebyte

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