[e2e] use of MAC addresses

Joe Touch touch at ISI.EDU
Wed Apr 12 11:13:39 PDT 2006

Hash: SHA1

Fahad Dogar wrote:
> On 4/12/06, Joe Touch <touch at isi.edu> wrote:
>> Fahad Dogar wrote:
>>> I understand why IP addresses (which are hierarchical in nature) are
>>> needed but can't seem to appreciate the use of MAC addresses, in
>>> addition to using IP addresses.
>> That's for the level of indirection Ted was noting.
>>> Practically I can understand that they
>>> are being used for legacy purposes and a move towards using IP
>>> addresses as layer 2 addresses would require changes in ethernet
>>> switches etc.
>> Another reason is that ethernet switches are  simpler to implement than
>>  (IP) routers. A final one is to limit broadcast; right now, broadcast
>> pervades (most) ethernet switches, but is halted by IP routers (even
>> subnet-directed broadcast is defaulted that way). It's not clear how to
>> allow broadcast to go more than one hop and control flooding (with
>> all-1's) or defeat DOS attacks (subnet broadcast).
> By using IP addresses for layer 2 routing, I am not implying the
> replacement of ethernet switches with routers. The ethernet protocol
> remains the same but the MAC address of every station is the same as
> its IP address. So these addresses have different interpretation (and
> subsequent forwarding) in a LAN compared to routing at layer 3. So we
> can block broadcast at layer 3 (routers) and allow broadcost at layer
> 2 (ethernet switches). Again, I am asking purely from the
> functionality point of view. It may not be practical to do such a
> thing.

With Ethernet, unique MACs are burned-in when a device is created.

If the L2 address matches the L3 address, that can't happen at device
creation time. How would you pick your first MAC address, e.g., to
bootstrap the device via DHCP?

If the L2 address need not match the L3, then there's no real reason not
to call it an IP address, but no reason to do so either.

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