[e2e] Fwd: Re: use of MAC addresses
detlef.bosau at web.de
Fri Apr 14 08:26:10 PDT 2006
> 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.
The second one is obsolete, as we dropped transparent segmentation in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.
Mail: detlef.bosau at web.de
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