[e2e] Fwd: Re: use of MAC addresses

Detlef Bosau detlef.bosau at web.de
Fri Apr 14 08:26:10 PDT 2006

weigengyu wrote:

> From the experience of telecom, separation of logical number and physical number is necessary;
> and separation of IP address function and Mac address is also necessary.

Basically, that´s the very question: Is the seperation of logical and 
physical addresses really _necessary_? Or ist it done for historical 

What I find particularly interesting in this discussion is, that it´s 
obviously difficult, to find compelling reasons for this separation.

In fact, when I read Craig´s remark on mobility, this was perhaps the 
most compelling reason for this separation in the whole discussion.

The reason for this might be, that IP is basically nothing else then a 
protocol for a "generic network" which is in fact quite similar to the 
interconnected "subnets". We establish an "internetwork" consisting on a 
number of "subnetworks".

Of course, some of the subnetworks in use have mechanisms which are not 
available to other ones, e.g. we can find upper limits for 
transportation times in Token Ring whereas the same task is not possible 
for Ethernet. If we combine those different subnetwokrss to an 
internetworks, the internetwork´s feature set is necessarily the 
intersection of the combined subnet´s ones.

Sometimes, I ask people for the difference between routing and bridging. 
And then I get long answers, even longer than one of my posts. However, 
the answer is astonishing simple: Basically, there is none.

In either case, a store and forward system receives a packet and has to 
decide where to send it.

Typically, IP routing is characterized by three tasks:
1. Addressing. Find a common address space for the whole network and a 
unique address for each member.
2. Segmentation and Reassembly.
3. Routing.

The second one is obsolete, as we dropped transparent segmentation in 
favour of do path MTU discovery.

The third one is basically the same on L2 and L3.

The first one is a matter of definition.

However, I said routing was _basically_ the same one. In fact, there are 
often subtle differences between routing in a LAN, in a WAN, in small 
networks with only a few nodes, in large networks with a huge number of 
nodes or in networks with mobile nodes which can be attached to the 
network in different position.

And of course, we did not talk about network specific requirements yet, 
like functional addresses or certain address formats for member 
solicitations, which are beyound the scope of IP.

If you look at an IP network and e.g. an Ethernet from a programmers 
point of view, the differences are sometimes very subtle. And this may 
be the reason for the confusion.

Detlef Bosau
Galileistrasse 30
70565 Stuttgart
Mail: detlef.bosau at web.de
Web: http://www.detlef-bosau.de
Mobile: +49 172 681 9937

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