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From "Mark H. Wood" <mw...@IUPUI.Edu>
Subject Re: [users@httpd]
Date Tue, 10 Nov 2009 19:31:26 GMT
On Tue, Nov 10, 2009 at 10:52:18AM -0500, Brian Mearns wrote:
> As Eric says, what you're looking for is a MAC address which is a
> universally unique identifier that every network device has (though I
> think even here, "universally unique" might have some qualifying
> conditions).

Yes.  Occasionally a manufacturer goofs and issues more than one part
with the same MAC address burned in.  We received two desktop boxes
with the same MAC address once.  Imagine how often it happens and
isn't caught, because they went to two different customers. :-(

Also, the IEEE 802 standards define Locally Administered Addresses.
Essentially you can tell most Ethernet adaptors to use any MAC address
you please, so long as a particular bit is on.*

A further complication is that at any time, someone could have a NIC
fail, replace it, and wind up using a different MAC address from then
on.  Is that one user, or two?  You don't know.

> MAC addresses are used in very low level protocols (link
> layer protocols, I believe) to send packets to specific devices. MAC
> addresses are for point-to-point communications, not end-to-end.

To make this plainer:  the first router on the path from user X to you
will discard this information.  As far as the protocol stack is
concerned, it has no useful meaning beyond that point.

It's thus very unlikely that you will get any help from hardware
manufacturers in identifying remote users or hosts in the manner you
specified.  You'll need cooperation from your users.

*  DECnet Phase IV used this to encode the network-layer address in
   the link-layer address, so the same NIC would have two different
   MAC addresses depending on whether DECnet had been started.  And
   the LAA would change if you ever changed the network-layer
   address.  What fun.

Mark H. Wood, Lead System Programmer   mwood@IUPUI.Edu
Friends don't let friends publish revisable-form documents.

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