Steve Deering wrote:
At 3:41 PM -0800 2/15/01, Ed Gerck wrote:
You give a name to your house (say, "The Tulip") and
the post office knows where The Tulip is. If you move,
you can do the same at your new location, provided
there is no conflict.> >
...Note that this is a natural example of NAT,
in which the post office is doing the address translation to a local
address that only that post office knows, but which is globally
reachable through that post office. And the post office does so
without changing the global addresses or the local addresses.
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.
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. 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
If another location is assigned to "The Tulip" (for example, because the owner
Mr. Tulip moved), the post office will deliver the original envelope there and
not at "545 Abbey St."
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".
In Internet NAT terms, "The Tulip" is the globally routable IP number for my
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. For my other hosts, I simply tell
the NAT box (post office) what is the local IP number that will receive the next
packet for "The Tulip" -- my single global name. If now you add a mailbox
"The Tulip" you have the same functionality of port translation as well, where
different local addresses (for private mail, for example) will correspond to
"n" in "The Tulip, PO Box n".
In other words, this is a natural NAT example and clearly supports the view
NATs are naturally occuring solutions to provide for local flexibility (Mr.
can change residence at will and can have more than one recipient for private
without decreasing global connectivity ("The Tulip" is always responsive).