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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 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 2 of 3)

Standard DNS-Based E-Mail Addresses

In TCP/IP, the system used for identifying servers (and other machines) is the Domain Name System (DNS). DNS is a big system and is described in its own large section in this Guide; I would refer you to it if you want to learn more. For our purposes, what is important is that in DNS, all devices on the Internet are arranged into a device naming hierarchy, and any device can be identified using a domain name consisting of a series of text labels separated by dots.

So, the complete TCP/IP address consists of two components: a user name specification and a domain name specification. The two are connected together using the special “at” symbol (“@”) to form the TCP/IP e-mail address syntax that almost all of us are familiar with today:

<username>@<domainname>

The format of <domainname> follows the syntax rules of DNS, containing only numbers and digits for each label, and periods to separate the labels. The format of <username> is slightly less restrictive, allowing the use of special characters such as the underscore (“_”). Other special characters and spaces are also allowed in the <username> if they are surrounded by quotes (or otherwise marked as being part the name, such as through the use of an escape character.) Domain names are case-insensitive; user names may be case-sensitive, depending on the system.

So, an example of a valid e-mail address might be the following address I used when I was in school many years ago:

cmk@athena.mit.edu

Where “cmk” is my user name (my initials) and the “athena.mit.edu” is the name of the host where I was receiving mail. “athena” is a particular system at MIT (“mit”), which is an educational institution, and so uses the “.edu” top-level domain.

URL E-Mail Address Specification

It is also possible to specify an e-mail address using an Internet-standard Uniform Resource Locator (URL). This allows a link to be embedded in a hypertext (Web) document that when clicked, invokes an e-mail client to send mail to a user. E-mail URLs are created by preceding the address by the special URL scheme string “mailto:”, like this:

mailto:cmk@athena.mit.edu

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TCP/IP Electronic Mail Addresses and Addressing
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TCP/IP Historical and Special Electronic Mail Addressing
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Version 3.0 - Version Date: September 20, 2005

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