TCP/IP Electronic Mail Addressing and Address Resolution
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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:
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:
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.
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:
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