At 6:21 PM -0800 2/15/01, Ed Gerck wrote:
Steve Deering wrote:
They also do it without removing the original destination address and
replacing it with another one -- the original envelope arrives at the
house with the destination address still saying "The Tulip", i.e., it
has not been translated, and thus is not analogous to NAT.
I think you got the example addresses reversed. In the case I mention,
"The Tulip" is the global address and (for the sake of example) suppose
now that "545 Abbey St." is the local physical address known to the post
office.
Yes, I understood that.
Thus, when the mailman delivers an envelope addressed to "The Tulip" at
"545 Abbey St.", that mailman is doing address translation -- and he may
even have written "545 Abbey St." on the envelope as a reminder.
No, he's doing address mapping, similar to the the mapping that is
done from an IP address to an Ethernet address to accomplish last-hop
delivery. The original, globally unique name (The Tulip, UK) is still
present on the letter. The local address may or may not also be present;
depending on whether or not "encapsulation" (i.e., adding on the
local address) was required to accomplish the delivery.
In the case of NAT, on the other hand, the destination address used
across the public part of the Internet is no longer present in the packet
finally delivered to the destination host -- it has been been replaced
by (i.e., translated to) a different address.
So, when the original envelope arrives at the destination address it
did so not because it had "The Tulip" written on it but because the post
office was able to do address translation to the *current* location which
is "545 Abbey St."
No, it was because they were able to do the mapping to the current
location. Translation, (i.e., replacing the address on the envelope
with another address) is not necessary and not done. The envelope may
well be *augmented* with an additional address, but the original
address is not removed.
Note that the local address which only the post office (and Mr. Tulip) knows
is "545 Abbey St." while the global address is "The Tulip".
The important point is that Mr. Tulip knows *both* addresses, and can
tell his international correspondents what his globally-unique address is.
A host behind a NAT, on the other hand, doesn't know its own global
address and, in most cases, doesn't even have a global address (or one
port's share of a global address), except temporarily as a side-effect
of sending a packet to the outside world.
In Internet NAT terms, "The Tulip" is the globally routable IP number for
my DSL, the post office is my NAT box and the physical address
"545 Abbey St." is the local, non-routable IP number of my host A.
That would be analogous to having "The Tulip, UK" be the address of
a post office, with all houses served by that post office sharing
the same global address of "The Tulip, UK". That indeed is like a
NAT, but is not the same as the original example.
In other words, this is a natural NAT example...
The original example, of a single house with the global address of
"The Tulip, UK" is a naturally occurring example of something like ARP
or something like tunneling, not something like NAT. The distinction
is betweeen doing a mapping/encapsulation and doing an address
substitution. NATs are all about doing address substitution; the
post office does mapping/encapsulation to deliver to The Tulip.
Steve