[e2e] use of MAC addresses

Joe Touch touch at ISI.EDU
Wed Apr 12 10:13:42 PDT 2006

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).

> But I am interested in knowing whether, in theory, we
> would be restricted in functionality if we use IP addresses for layer
> 2 addressing. Suppose we were to redesign layer 2 technologies now,
> can we use IP addresses in place of MAC addresses. With IPv6 I can't
> even see any constraint on the address space.

IPv6 addresses may seem large at first, but given how they're
hierarchically delegated and managed, they're not all that big.


> Thanks,
> Fahad
> On 4/12/06, Joe Touch <touch at isi.edu> wrote:
>> Ted Faber wrote:
>>> On Tue, Apr 11, 2006 at 12:20:47AM +0500, Fahad Dogar wrote:
>>>> Hi all,
>>>> I have a very basic question: in theory, can we NOW use IP addresses
>>>> in place of MAC addresses.
>>> Short answer: "yes," with an "if."
>>> Long answer: "no," with a "but."
>>> IP addresses are (in principle) globally routable.
>>                                 ^
>>                              easily
>> You can route on MAC addresses too, but being flat that means core
>> routing tables would need to be flooded with everyone's MAC address.
>> Look at the size of your routing table. Then look at the size of your
>> ARP table. If your routing table has anything except default addresses,
>> consider that ARP table size multipled by the size of the number of
>> subnets at each other route entry.
>> Joe
>>  Having a locally
>>> routable namespace under your link layer's complete control may be a
>>> useful thing.
>>> It's rare in the world of computers that removing a layer of indirection
>>> makes your system more versatile.  There are quite a few tricks that
>>> take advantage of the layer of indirection that a link layer address
>>> provides to give faster response on a subnet basis, simple redundancy,
>>> etc.
>>> Of course you could get rid of them (assuming you're willing to live in
>>> the smaller, more constained IP address space).  An identifier's an
>>> identifier.
>>> Why would you want to?

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