87 lines
3.8 KiB
HTML
87 lines
3.8 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>Subnetworks</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="node30.html">Gateways</A>
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<B>Up:</B> <A HREF="node27.html">IP Routing</A>
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<B> Previous:</B> <A HREF="node28.html">IP Networks</A>
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<BR> <P>
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<H2><A NAME="SECTION004420000">Subnetworks</A></H2>
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This structure is reflected by splitting IP-addresses into a host and
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network part, as explained above. By default, the destination network is
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derived from the network part of the IP-address. Thus, hosts with
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identical IP-network numbers should be found within the same network,
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and vice versa.<A HREF="footnode.html#877"><IMG ALIGN=BOTTOM ALT="gif" SRC="foot_motif.gif"></A>
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<P>
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It makes sense to offer a similar scheme <em>inside</em> the network,
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too, since it may consist of a collection of hundreds of smaller
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networks itself, with the smallest units being physical networks like
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Ethernets. Therefore, IP allows you to subdivide an IP-network into
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several <em>subnets</em>.
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<P>
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A subnet takes over responsibility for delivering datagrams to a certain
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range of IP-addresses from the IP-network it is part of. As with
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classes A, B, or C, it is identified by the network part of the
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IP-addresses. However, the network part is now extended to include
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some bits from the host part. The number of bits that are interpreted
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as the subnet number is given by the so-called <em>subnet mask</em>, or
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<em>netmask</em>. This is a 32-bit number, too, which specifies the bit
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mask for the network part of the IP-address.
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<P>
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<P><A NAME="1007"></A><BR>
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<STRONG>Figure:</STRONG>
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<A NAME="tcpipfigsubnet"></A>
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Subnetting a class B network
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<!--tony: working on it-->
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<BR>
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<P>
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<P>
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<A NAME="940"></A>
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The campus network of Groucho Marx University is an example of such a
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network. It has a class-B network number of 149.76.0.0, and its
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netmask is therefore 255.255.0.0.
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<P>
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Internally, GMU's campus network consists of several smaller networks,
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such as the LANs of various departments. So the range of IP-addresses
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is broken up into 254 subnets, 149.76.1.0 through
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149.76.254.0. For example, the Department of Theoretical
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Physics has been assigned 149.76.12.0. The campus backbone is
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a network by its own right, and is given 149.76.1.0. These
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subnets share the same IP-network number, while the third octet is
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used to distinguish between them. Thus they will use a subnet mask of
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255.255.255.0.
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<P>
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Figure <A HREF="node29.html#tcpipfigsubnet"><IMG ALIGN=BOTTOM ALT="gif" SRC="cross_ref_motif.gif"></A> shows how 149.76.12.4, the
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address of quark, is interpreted differently when the address
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is taken as an ordinary class-B network, and when used with
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sub-netting.
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<P>
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<A NAME="951"></A>
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<A NAME="952"></A>
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It is worth noting that sub-netting (as the technique of generating
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subnets is called) is only an <em>internal division</em> of the network.
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Subnets are generated by the network owner (or the administrators).
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Frequently, subnets are created to reflect existing boundaries, be they
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physical (between two Ethernets), administrative (between two
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departments), or geographical, and authority over these subnets is
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delegated to some contact person. However, this structure affects only
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the network's internal behavior, and is completely invisible to the
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outside world.
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<P>
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<A NAME="954"></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="node30.html">Gateways</A>
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<B>Up:</B> <A HREF="node27.html">IP Routing</A>
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<B> Previous:</B> <A HREF="node28.html">IP Networks</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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