114 lines
5.2 KiB
HTML
114 lines
5.2 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>IP 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="node26.html">Address Resolution</A>
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<B>Up:</B> <A HREF="node23.html">Issues of TCP/IP Networking</A>
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<B> Previous:</B> <A HREF="node24.html">Networking Interfaces</A>
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<BR> <P>
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<H1><A NAME="SECTION004200000">IP Addresses</A></H1>
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As mentioned in the previous chapter, the addresses understood by the
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IP-networking protocol are 32-bit numbers. Every machine must be assigned a
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number unique to the networking environment. If you are running
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a local network that does not have TCP/IP traffic with other
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networks, you may assign these numbers according to your personal
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preferences. However, for sites on the Internet, numbers are
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assigned by a central authority, the Network Information Center,
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or NIC.<A HREF="footnode.html#891"><IMG ALIGN=BOTTOM ALT="gif" SRC="foot_motif.gif"></A>
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<P>
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<A NAME="812"></A>
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For easier reading, IP addresses are split up into four 8-bit numbers
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called <em>octets</em>. For example, quark.physics.groucho.edu has an
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IP-address of 0x954C0C04, which is written as 149.76.12.4.
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This format is often referred to as the <em>dotted quad notation</em>.
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<P>
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Another reason for this notation is that IP-addresses are split into a
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<em>network</em> number, which is contained in the leading octets, and a
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<em>host</em> number, which is the remainder. When applying to the NIC
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for IP-addresses, you are not assigned an address for each single host
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you plan to use. Instead, you are given a network number, and are
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allowed to assign all valid IP-addresses within this range to hosts on
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your network according to your preferences.
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<P>
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Depending on the size of the network, the host part may need to be
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smaller or larger. To accommodate different needs, there are several
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classes of networks, defining different splits of IP-addresses.
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<pre>
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Class A Class A comprises networks 1.0.0.0 through 127.0.0.0. The
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network number is contained in the first octet. This provides
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for a 24 bit host part, allowing roughly 1.6 million hosts.
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Class B Class B contains networks 128.0.0.0 through 191.255.0.0; the
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network number is in the first two octets. This allows for
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16320 nets with 65024 hosts each.
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Class C Class C networks range from 192.0.0.0 through 223.255.255.0,
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with the network number being contained in the first three
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octets. This allows for nearly 2 million networks with up to
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254 hosts.
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Classes D, E, and F Addresses falling into the range of 224.0.0.0
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through 254.0.0.0 are either experimental, or are reserved for
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future use and don't specify any network.
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</pre>
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If we go back to the example in the previous chapter, we find that
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149.76.12.4, the address of quark, refers to host
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12.4 on the class-B network 149.76.0.0.
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<P>
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<A NAME="834"></A>
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You may have noticed that in the above list not all possible values were
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allowed for each octet in the host part. This is because host numbers
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with octets all 0 or all 255 are reserved for special purposes.
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An address where all host part bits are zero refers to the network, and
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one where all bits of the host part are 1 is called a broadcast address.
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This refers to all hosts on the specified network simultaneously.
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Thus, 149.76.255.255 is not a valid host address, but refers
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to all hosts on network 149.76.0.0.
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<P>
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<A NAME="839"></A>
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<A NAME="892"></A>
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<A NAME="841"></A>
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<A NAME="842"></A>
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There are also two network addresses that are reserved, 0.0.0.0 and
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127.0.0.0. The first is called the <em>default route</em>, the latter
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the <em>loopback address</em>. The default route has something to do with the
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way IP routes datagrams, which will be dealt with below.
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<P>
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Network 127.0.0.0 is reserved for IP traffic local to your
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host. Usually, address 127.0.0.1 will be assigned to a special
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interface on your host, the so-called <em>loopback interface</em>, which
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acts like a closed circuit. Any IP packet handed to it from TCP or UDP
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will be returned to them as if it had just arrived from some network.
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This allows you to develop and test networking software without ever
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using a ``real'' network. Another useful application is when you want
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to use networking software on a standalone host. This may not be as
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uncommon as it sounds; for instance, many UUCP sites don't have IP
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connectivity at all, but still want to run the INN news system
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nevertheless. For proper operation on , INN requires the
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loopback interface.
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<P>
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<A NAME="851"></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="node26.html">Address Resolution</A>
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<B>Up:</B> <A HREF="node23.html">Issues of TCP/IP Networking</A>
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<B> Previous:</B> <A HREF="node24.html">Networking Interfaces</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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