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11.3. Email Addresses
Email addresses are made up of at least
two parts. One part is the name of a mail
domain that will ultimately translate to either
the recipient's host or some host that accepts mail
on behalf of the recipient. The other part is some form of unique
user identification that may be the login name of that user, the real
name of that user in
"Firstname.Lastname" format, or an
arbitrary alias that will be translated into a user or list of users.
Other mail addressing schemes, such as X.400, use a more general set
of "attributes" that are used to
look up the recipient's host in an X.500 directory
server.How email addresses are interpreted depends greatly on what type of
network you use. We'll concentrate on how TCP/IP
networks interpret email addresses.
11.3.1. RFC 822
Internet sites adhere to the RFC 822
standard, which requires the familiar notation of sign, but it helps if you read it as
"at." This notation does not
specify a route to the destination host. Routing of the mail message
is left to the mechanisms we'll describe shortly.
11.3.2. Obsolete Mail Formats
Before moving on, let's
have a look at the way things used to be. In the original UUCP
environment, the prevalent form was
path!host!user, for which
path described a sequence of hosts the message
had to travel through before reaching the destination
host. This construct is called the
bang path notation because
an exclamation mark is colloquially called a
"bang." Other networks had still different means
of addressing. DECnet-based networks, for example, used two colons as
an address separator, yielding an address of
host::user. The X.400 standard uses an entirely
different scheme, describing a recipient by a set of attribute-value
pairs, such as country and organization.Lastly, on FidoNet, each user was
identified by a code such as 2:320/204.9,
consisting of four numbers denoting zone (2 for Europe), net (320
referred to Paris and Banlieue), node (the local hub), and point (the
individual user's PC). Fidonet addresses were mapped
to RFC 822; the above, for example, was written as
