Summary: Locate real Ether address

From: Andrew Watkins (
Date: Tue Dec 03 1996 - 11:50:07 CST

Well thanks again to all who replied.

I looks like that there is a mixture of theories out there concerning
the fact that should you have different MAC addresses on the same host.
This is all because Sun believe that you should only have one
ethernet address for a WorkStation forgetting how many interfaces you have.
They store it on the PROM!

>From the Sun Managers FAQ:
12. Networking
Subject: 12.1) Why do both my net interfaces have the same ethernet

  The Ethernet version 2.0 specification (November 1982) states:

          The physical address of each station is set by network
          management to a unique value associated with the station,
          and distinct from the address of any other station on any
          Ethernet. The setting of the station's physical address
          by network management allows multiple multiple data link
          controllers connected to a single station to respond to
          the same physical address.

  This doesn't normally constitute a problem because each interface will
  typically be on a different subnet. If, for some reason, different
  ethernet addresses are required on different interfaces (for example,
  to attach two interfaces to the same subnet), a new one may be assigned
  using the ifconfig command.

I did examine to see if there was more packets on the network because
of the above, but did not see any increase. Pings were being doubled thou.

Anyway, I decided to have a different MAC address on my second interface
just to make sure - since we do not control all the routers here and some of them
were playing up. I got the ethernet address of an on PC card which I then snapped!
        % ifconfig le1 ether aa:1:2:3:4:5

        1) If you have separate subnets you do not need to have different MAC
        2) If you have one network it is best to have different MAC addresses
        3) If you are using switched ethernet then you have to.


Andrew Watkins tel: 0171-631 6720
Unix Administrator fax: 0171-631 6727
System Group
Computer Science Department e-mail:
Birkbeck College (University of London)
Malet Street

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