101 lines
4.9 KiB
HTML
101 lines
4.9 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>The Internet Protocol</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="node12.html">IP over Serial Lines</A>
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<B>Up:</B> <A HREF="node7.html">TCP/IP Networks</A>
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<B> Previous:</B> <A HREF="node10.html">Other Types of Hardware</A>
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<BR> <P>
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<H2><A NAME="SECTION003340000">The Internet Protocol</A></H2>
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<A NAME="introtcpipip"></A>
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Of course, you wouldn't want your networking to be limited to one
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Ethernet. Ideally, you would want to be able to use a network regardless
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of what hardware it runs on and how many subunits it is made up of. For
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example, in larger installations such as Groucho Marx University, you
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usually have a number of separate Ethernets that have to be connected in
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some way. At GMU, the maths department runs two Ethernets: one network
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of fast machines for professors and graduates, and another one with slow
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machines for students. Both are linked to the FDDI campus backbone.
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<P>
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This connection is handled by a dedicated host, a so-called <em>gateway</em>,
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which handles incoming and outgoing packets by copying them between the two
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Ethernets and the fiber optics cable. For example, if you are at the Maths
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Department, and want to access quark on the Physics Department's LAN
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from your box, the networking software cannot send packets to
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quark directly, because it is not on the same Ethernet. Therefore,
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it has to rely on the gateway to act as a forwarder. The gateway (name it
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sophus) then forwards these packets to its peer gateway niels
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at the Physics Department, using the backbone, with niels delivering
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it to the destination machine. Data flow between erdos and
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quark is shown in figure-<A HREF="node11.html#introfigipflow"><IMG ALIGN=BOTTOM ALT="gif" SRC="cross_ref_motif.gif"></A> (With apologies to
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Guy L. Steele).
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<P>
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<P><A NAME="732"></A><BR>
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<STRONG>Figure:</STRONG>
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<A NAME="introfigipflow"></A>
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The three steps of sending a datagram from erdos
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to quark.
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<BR>
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sorry, working on it (tony :()
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<P>
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<P>
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<A NAME="481"></A>
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This scheme of directing data to a remote host is called <em>routing</em>,
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and packets are often referred to as <em>datagrams</em> in this context.
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To facilitate things, datagram exchange is governed by a single protocol
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that is independent of the hardware used: IP, or <em>Internet
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Protocol</em>. In chapter-<A HREF="node23.html#tcpip"><IMG ALIGN=BOTTOM ALT="gif" SRC="cross_ref_motif.gif"></A>, we will cover IP and the issues of
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routing in greater detail.
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<P>
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<A NAME="486"></A>
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<A NAME="733"></A>
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<A NAME="488"></A>
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The main benefit of IP is that it turns physically dissimilar
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networks into one apparently homogeneous network. This is called
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internetworking, and the resulting ``meta-network'' is called
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an <em>internet</em>. Note the subtle difference between <em>an</em>
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internet and <em>the</em> Internet here. The latter is the official
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name of one particular global internet.
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<P>
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<A NAME="492"></A>
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<A NAME="493"></A>
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<A NAME="494"></A>
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Of course, IP also requires a hardware-independent addressing scheme. This
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is achieved by assigning each host a unique 32-bit number, called the
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<em>IP-address</em>. An IP-address is usually written as four decimal numbers,
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one for each 8-bit portion, separated by dots. For example, quark
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might have an IP-address of 0x954C0C04, which would be written as
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149.76.12.4. This format is also called <em>dotted quad</em> notation.
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<P>
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<A NAME="500"></A>
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<A NAME="501"></A>
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You will notice that we now have three different types of addresses: first
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there is the host's name, like quark, then there are IP-addresses,
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and finally, there are hardware addresses, like the 6-byte Ethernet
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address. All these somehow have to match, so that when you type
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rlogin quark, the networking software can be given quark's
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IP-address; and when IP delivers any data to the Physics Department's
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Ethernet, it somehow has to find out what Ethernet address corresponds to
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the IP-address. Which is rather confusing.
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<P>
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We will not go into this here, and deal with it in chapter-<A HREF="node23.html#tcpip"><IMG ALIGN=BOTTOM ALT="gif" SRC="cross_ref_motif.gif"></A>
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instead. For now, it's enough to remember that these steps of finding
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addresses are called <em>hostname resolution</em>, for mapping host names onto
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IP-addresses, and <em>address resolution</em>, for mapping the latter to
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hardware addresses.
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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="node12.html">IP over Serial Lines</A>
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<B>Up:</B> <A HREF="node7.html">TCP/IP Networks</A>
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<B> Previous:</B> <A HREF="node10.html">Other Types of Hardware</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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