Using arp and ifconfig alias

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From - Sat Apr 10 06:59:01 1999
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From: spcecdt@deeptht.armory.com. (John DuBois)
Newsgroups: comp.unix.sco.misc
Subject: Re: <> IP on same NIC , possible ?
Date: Sat, 10 Apr 1999 08:49:26 GMT
Organization: The Armory
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In article <7elv2t$t37$1@hobbes.caldera.com>, Evan Hunt <evanh@sco.COM> wrote:
>
>Don't blame me, I voted for spcecdt@deeptht.armory.com. (John DuBois).
>>>...
>>Hmmm?  He wants two IP addresses on the *same* NIC.  That's one mac address.
>
>That's true; the reason given for running arp was incorrect... but
>actually, it may still be necessary to run arp.
>
>Suppose you're on a class C network, 1.2.3.xxx.  Your system is named
>1.2.3.4, and you want it to respond to another IP address as well.
>
>If the new IP address is *also* going to be on the 1.2.3 subnet--
>say 1.2.3.5--then you need to run arp to publish another IP address
>for your existing mac address.

My understanding has always been that you only need to run arp if you want to
publish an IP address that *isn't* an address for an interface.
ifconfig -alias binds an IP address to an interface in such a way that it *is*
an address for the interface, so modification of the arp table with arp(ADMN)
isn't neccessary.
That understanding is borne out by my experience with IP aliases; I have a half
dozen IP aliases for addresses in the same class C network on my 5.0.0 system,
and have never done anything other than 'ifconfig alias' to establish them. 
The other systems on the thinnet as well as the DSL router (and the ISDN bridge
that preceded it) have had no problem seeing all of the IP addresses.

        John
-- 
John DuBois    spcecdt@armory.com.    KC6QKZ   http://www.armory.com./~spcecdt/

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