Please Whitelist This Site?

I know everyone hates ads. But please understand that I am providing premium content for free that takes hundreds of hours of time to research and write. I don't want to go to a pay-only model like some sites, but when more and more people block ads, I end up working for free. And I have a family to support, just like you. :)

If you like The TCP/IP Guide, please consider the download version. It's priced very economically and you can read all of it in a convenient format without ads.

If you want to use this site for free, I'd be grateful if you could add the site to the whitelist for Adblock. To do so, just open the Adblock menu and select "Disable on tcpipguide.com". Or go to the Tools menu and select "Adblock Plus Preferences...". Then click "Add Filter..." at the bottom, and add this string: "@@||tcpipguide.com^$document". Then just click OK.

Thanks for your understanding!

Sincerely, Charles Kozierok
Author and Publisher, The TCP/IP Guide


NOTE: Using software to mass-download the site degrades the server and is prohibited.
If you want to read The TCP/IP Guide offline, please consider licensing it. Thank you.

The Book is Here... and Now On Sale!

Get The TCP/IP Guide for your own computer.
The TCP/IP Guide

Custom Search







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

Previous Topic/Section
TCP/IP Electronic Mail Addresses and Addressing
Previous Page
Pages in Current Topic/Section
1
2
3
Next Page
TCP/IP Historical and Special Electronic Mail Addressing
Next Topic/Section

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

Previous Topic/Section
TCP/IP Electronic Mail Addresses and Addressing
Previous Page
Pages in Current Topic/Section
1
2
3
Next Page
TCP/IP Historical and Special Electronic Mail Addressing
Next Topic/Section

If you find The TCP/IP Guide useful, please consider making a small Paypal donation to help the site, using one of the buttons below. You can also donate a custom amount using the far right button (not less than $1 please, or PayPal gets most/all of your money!) In lieu of a larger donation, you may wish to consider purchasing a download license of The TCP/IP Guide. Thanks for your support!
Donate $2
Donate $5
Donate $10
Donate $20
Donate $30
Donate: $



Home - Table Of Contents - Contact Us

The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005

© Copyright 2001-2005 Charles M. Kozierok. All Rights Reserved.
Not responsible for any loss resulting from the use of this site.