[e2e] use of MAC addresses

Zartash Afzal Uzmi zartash at lums.edu.pk
Tue Apr 18 14:19:26 PDT 2006

Detlef Bosau wrote:
> ...
> In principle, you can interconnect all computers in the world via a flat
> Ethernet. (Define a "DS-Ethernet" := Deep Sea Ethernet for
> intercontinental lines if necessary.)
> How large will the switching tables be?
> Can we extend this until the switching tables grow beyond all limits?
> And so does the processing time on routers?

If we do consider a "DS-Ethernet", switching tables (whatever they mean)
will explode and processing times will be too long. However, this was not
the intent of the original post. I see some people in the discussion are
still concerned about table entries and scalability. Let me try to phrase
the original question, in an attempt to bring more clarity:

Consider a node that uses two identifiers: an ethernet address (call it E)
and an IPv4 address (call it I). Theoretically, do we need two separate
identifiers? In other words, can these two identifiers I and E be equal?
Please note that this does not mean that any change is proposed to the way
forwarding is done in either LAN or WAN.

Lets assume we have one single identifier (let me call it X). Now, LAN
devices have exactly the same forwarding tables as before (when we were
using two identifiers E and I) except that E is replaced by X. Similarly,
WAN devices (routers) also do not have anything different from the previous
case (when two separate identifiers were used) except that I is replaced by
X in all the table entries. Absolutely, no change in table size!

This is possible, at least in theory. First we assume that each node somehow
knows (or gets) its IP address which is nothing more than a sequence of
bits. Then the network card is made to use the same identifier as the
ethernet address. While this seems theoretically possible, one real
"technical" issue would be how a node will get the IP address in the first
place? For DHCP to work as it does today, one has to has an ethernet address
a priori. If everyone harcodes the IP (again, only a theoretical whim), we
are okay and can use same identifier as the ethernet address too; otherwise
we may use some other mechanism to first acquire IP address (without having
an ethernet address). The issue of initial acquisition of IP address has
been pointed out by some people (refer to the emails by Joe and Craig,

Is inital acquisition of IP address the only issue? I am not sure but I find
no other "theoretical" reason in the discussion so far. Well, with the
frequency of posts on this topic, other reasons may have appeared on the
list while I wrote this email :) But obviously, two unique identifiers,
ethernet and IP, work. There does not seem any apparent practical reason one
should use the same identifier for both just for the sake of saving some
identifiers that will not be used anyways!


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