147 lines
6.5 KiB
HTML
147 lines
6.5 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>Enter DNS</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="node37.html">Name Lookups with DNS</A>
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<B>Up:</B> <A HREF="node34.html">The Domain Name System</A>
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<B> Previous:</B> <A HREF="node35.html">Hostname Resolution</A>
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<BR> <P>
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<H2><A NAME="SECTION004620000">Enter DNS</A></H2>
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DNS organizes host names in a hierarchy of domains. A domain is a
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collection of sites that are related in some sense--- be it because
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they form a proper network (e.g. all machines on a campus, or all hosts
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on BITNET), because they all belong to a certain organization (like the
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U.S. government), or because they're simply geographically close. For
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instance, universities are grouped in the edu domain, with each
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University or College using a separate <em>subdomain</em> below which
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their hosts are subsumed. Groucho Marx University might be given the
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groucho.edu domain, with the LAN of the Mathematics Department
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being assigned maths.groucho.edu. Hosts on the departmental
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network would have this domain name tacked onto their host name; so
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erdos would be known as erdos.maths.groucho.edu. This is
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called the <em>fully qualified domain name</em>, or FQDN, which uniquely
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identifies this host world-wide.
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<P>
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<P><A NAME="1745"></A><BR>
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<STRONG>Figure:</STRONG>
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<A NAME="introfigdns"></A>
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A part of the domain name space
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<BR>Another lost figure?
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<P>
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<P>
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Figure-<A HREF="node36.html#introfigdns"><IMG ALIGN=BOTTOM ALT="gif" SRC="cross_ref_motif.gif"></A> shows a section of the name space. The entry
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at the root of this tree, which is denoted by a single dot, is quite
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appropriately called the <em>root domain</em>, and encompasses all other
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domains. To indicate that a host name is a fully qualified domain name,
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rather than a name relative to some (implicit) local domain, it is
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sometimes written with a trailing dot. This signifies that the name's
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last component is the root domain.
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<P>
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<A NAME="1530"></A>
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Depending on its location in the name hierarchy, a domain may be
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called top-level, second-level, or third-level. More levels of
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subdivision occur, but are rare. These are a couple of top-level
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domains you may see frequently:
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<pre>
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edu (Mostly US) educational institutions like universities, etc.
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com Commercial organizations, companies.
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org Non-commercial organizations. Often private UUCP networks are
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in this domain.
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net Gateways and other administrative host on a network.
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mil US military institutions.
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gov US government institutions.
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uucp Officially, all site names formerly used as UUCP names without
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domain, have been moved to this domain.
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</pre>
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Technically, the first four of these belong to the US part of the
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Internet, but you may also see non-US sites in these domains. This is
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especially true of the net domain. However, mil
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and gov are used exclusively in the US.
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<P>
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Outside the US, each country generally uses a top-level domain of its
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own named after the two-letter country code defined in ISO-3166.
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Finland, for instance, uses the fi domain, fr is used by
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France, de by Germany, or au by Australia etc. Below
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this top-level domain, each country's NIC is free to organize host
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names in whatever way they want. Australia, for example, has
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second-level domain similar to the international top-level domains,
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named com.au, edu.au, and so on. Others, like Germany,
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don't use this extra level, but rather have slightly longish names that
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refer directly to the organizations running a particular domain. For
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example, it's not uncommon to see host names like
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ftp.informatik.uni-erlangen.de. Chalk that up to German
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efficiency.
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<P>
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Of course, these national domains do not imply that a host below that
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domain is actually located in that country; it only signals that the
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host has been registered with that country's NIC. A Swedish manufacturer
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might have a branch in Australia, and still have all its hosts
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registered with the se top-level domain.
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<P>
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Now, organizing the name space in a hierarchy of domain names nicely
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solves the problem of name uniqueness; with DNS, a host name has to be
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unique only within its domain to give it a name different from all
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other hosts world-wide. Furthermore, fully qualified names are quite
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easy to remember. Taken by themselves, these are already very good
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reasons to split up a large domain into several subdomains.
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<P>
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<A NAME="1551"></A>
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<A NAME="1552"></A>
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<A NAME="1553"></A>
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But DNS does even more for you than than this: it allows you to delegate
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authority over a subdomain to its administrators. For example, the
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maintainers at the Groucho Computing Center might create a subdomain for
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each department; we already encountered the maths and
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physics subdomains above. When they find the network at the
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Physics Department too large and chaotic to manage from outside (after
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all, physicists are known to be an unruly bunch of people), they may
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simply pass control over the physics.groucho.edu domain to the
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administrators of this network. These are then free to use whatever
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host names they like, and assign them IP addresses from their network in
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whatever fashion the like, without outside interference.
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<P>
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<A NAME="1557"></A>
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<A NAME="1746"></A>
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To this end, the name space is split up into <em>zones</em>, each rooted
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at a domain. Note the subtle difference between a zone and a domain:
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the <em>domain</em> groucho.edu encompasses all hosts at the
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Groucho Marx University, while the <em>zone</em> groucho.edu
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includes only the hosts that are managed by the Computing Center
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directly, for example those at the Mathematics Department. The hosts
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at the Physics Department belong to a different zone, namely
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physics.groucho.edu. In figure-<A HREF="node36.html#introfigdns"><IMG ALIGN=BOTTOM ALT="gif" SRC="cross_ref_motif.gif"></A>, the start
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of a zone is marked by a small circle to the right of the domain name.
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<A NAME="1566"></A>
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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="node37.html">Name Lookups with DNS</A>
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<B>Up:</B> <A HREF="node34.html">The Domain Name System</A>
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<B> Previous:</B> <A HREF="node35.html">Hostname Resolution</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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