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Table Of Contents  The TCP/IP Guide
 9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)
      9  TCP/IP Key Applications and Application Protocols
           9  TCP/IP File and Message Transfer Applications and Protocols (FTP, TFTP, Electronic Mail, USENET, HTTP/WWW, Gopher)
                9  TCP/IP Electronic Mail System: Concepts and Protocols (RFC 822, MIME, SMTP, POP3, IMAP)
                     9  TCP/IP Electronic Mail Addresses and Addressing

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TCP/IP Historical and Special Electronic Mail Addressing
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TCP/IP Electronic Mail Addressing and Address Resolution
(Page 1 of 3)

All communication on an internetwork requires some way of specifying the identify of the intended recipient of the communication. Most application protocols, such as FTP and HTTP for example, use conventional TCP/IP constructs—IP addresses and port numbers—to specify the destination of information to be sent. The IP address normally identifies a particular host computer, and the port number a software process or application running on that computer.

Electronic mail, however, uses a very different model for communication than most applications. As we saw in our discussion of the e-mail model, one element that sets e-mail apart from many other systems is that communication is user-oriented. E-mail is not sent from one machine to another the way a file is transferred using FTP. It is sent from one user to another. This is critical to the operation of the entire system; for one thing, it is what allows someone to retrieve e-mail that has been sent from any number of different client computers. This allows e-mail to be received even when traveling, for example.

Since e-mail messaging is user-based, it is necessary that the addressing scheme be user-based as well. We cannot use conventional IP addresses and ports, but need a distinct system that specifies two primary pieces of information: who the user is, and where the user is located. These are, of course, analogous to a name and address on a regular mail envelope.

The idea of a user name is relatively straight-forward, but identifying the location of the user is not. In regular mail, an address refers to a physical place. It would have been possible to define e-mail addresses in the same way: have it refer to the user's client machine. However, recall the other important characteristic of e-mail delivery: it is indirect, and based on the concept of a user's local SMTP server holding received messages until they can be retrieved. The machine that the user employs to access his or her e-mail may not even routinely be on the Internet, and it may thus not be easy to identify it. And again, we want a user to be able to access mail from multiple machines.

For all of these reasons, we want addresses to identify not the user's specific location at any particular time, but the place where the user's permanent mailbox lives. This is on the user's SMTP server, which is permanently connected to the Internet.


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