100 lines
4.7 KiB
HTML
100 lines
4.7 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN">
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<!--Converted with LaTeX2HTML 96.1-c (Feb 29, 1996) by Nikos Drakos (nikos@cbl.leeds.ac.uk), CBLU, University of Leeds -->
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<HTML>
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<HEAD>
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<TITLE>Email Addresses</TITLE>
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</HEAD>
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<BODY LANG="EN">
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<A HREF="node1.html"><IMG WIDTH=65 HEIGHT=24 ALIGN=BOTTOM ALT="contents" SRC="contents_motif.gif"></A> <BR>
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<B> Next:</B> <A HREF="node190.html">How does Mail Routing </A>
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<B>Up:</B> <A HREF="node186.html">Electronic Mail</A>
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<B> Previous:</B> <A HREF="node188.html">How is Mail Delivered?</A>
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<BR> <P>
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<H1><A NAME="SECTION0015300000">Email Addresses</A></H1>
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<A NAME="mailaddress"></A>
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For electronic mail, an address is made up of at least the name of a
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machine handling the person's mail, and a user identification recognized
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by this system. This may be the recipient's login name, but may also be
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anything else. Other mail addressing schemes, like X.400, use a more
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general set of ``attributes'' which are used to look up the recipient's
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host in an X.500 directory server.
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<P>
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The way a machine name is interpreted, i.e. at which site your message
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will finally wind up, and how to combine this name with the recipient's
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user name greatly depends on the network you are on.
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<P>
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Internet sites adhere to the RFC-822 standard, which requires a notation
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of user@host.domain, where host.domain is the host's fully
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qualified domain name. The middle thing is called an ``at'' sign.
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Because this notation does not involve a route to the destination host
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but gives the (unique) hostname instead, this is called an <em>absolute</em>
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address.
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<P>
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In the original UUCP environment, the prevalent form was
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path!host!user, where path described a sequence of hosts
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the message had to travel before reaching the destination host.
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This construct is called the <em>bang path</em> notation, because an
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exclamation mark is loosely called a ``bang''. Today, many UUCP-based
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networks have adopted RFC-822, and will understand this type of
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address.
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<P>
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Now, these two types of addressing don't mix too well. Assume an address
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of hostA!user@hostB. It is not clear whether the `@' sign
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takes precedence over the path, or vice versa: do we have to send the
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message to hostB, which mails it to hostA!user, or should it
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be sent to hostA, which forwards it to user@hostB?
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<P>
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Addresses that mix different types of address operators are called
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<em>hybrid addresses</em>. Most notorious is the above example. It is
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usually resolved by giving the `@' sign precedence over the
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path. In the above example, this means sending the message to
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hostB first.
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<P>
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However, there is a way to specify routes in RFC-822-conformant ways:
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<@hostA,@hostB:user@hostC> denotes the address of user on
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hostC, where hostC is to be reached through hostA
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and hostB (in that order). This type of address is frequently
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called a <em>route-addr address</em>.
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<P>
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Then, there is the `%' address operator: user%hostB@hostA
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will first be sent to hostA, which expands the rightmost (in this
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case, only) percent sign to an `@' sign. The address is now
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user@hostB, and the mailer will happily forward your message to
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hostB which delivers it to user. This type of address is
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sometimes referred to as ``Ye Olde ARPANET Kludge'', and its use is
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discouraged. Nevertheless, many mail transport agents generate this
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type of address.
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<P>
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Other networks have still different means of addressing. DECnet-based
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networks, for example, use two colons as an address separator, yielding
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an address of host::user.<A HREF="footnode.html#7189"><IMG ALIGN=BOTTOM ALT="gif" SRC="foot_motif.gif"></A> Lastly, the X.400 standard uses an entirely different scheme, by
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describing a recipient by a set of attribute-value pairs, like country
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and organization.
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<P>
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On FidoNet, each user is identified by a code like 2:320/204.9,
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consisting of four numbers denoting zone (2 is for Europe), net (320
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being Paris and Banlieue), node (the local hub), and point (the
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individual user's PC). Fidonet addresses can be mapped to RFC-822; the
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above would be written as
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Thomas.Quinot@p9.f204.n320.z2.fidonet.org. Now didn't I say
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domain names are easy to remember?
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<P>
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There are some implications to using these different types of addressing
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which will be described throughout the following sections. In a RFC-822
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environment, however, you will rarely use anything else than absolute
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addresses like user@host.domain.
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<P>
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<A NAME="6929"></A>
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<A NAME="6930"></A>
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<P>
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<HR><A HREF="node1.html"><IMG WIDTH=65 HEIGHT=24 ALIGN=BOTTOM ALT="contents" SRC="contents_motif.gif"></A> <BR>
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<B> Next:</B> <A HREF="node190.html">How does Mail Routing </A>
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<B>Up:</B> <A HREF="node186.html">Electronic Mail</A>
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<B> Previous:</B> <A HREF="node188.html">How is Mail Delivered?</A>
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<P><ADDRESS>
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<I>Andrew Anderson <BR>
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Thu Mar 7 23:22:06 EST 1996</I>
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</ADDRESS>
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</BODY>
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</HTML>
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