114 lines
3.6 KiB
HTML
114 lines
3.6 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN">
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<HTML>
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<HEAD>
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<META NAME="GENERATOR" CONTENT="SGML-Tools 1.0.9">
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<TITLE>IP Sub-Networking Mini-Howto: Routing</TITLE>
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<LINK HREF="IP-Subnetworking-6.html" REL=previous>
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<LINK HREF="IP-Subnetworking.html#toc7" REL=contents>
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</HEAD>
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<BODY>
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Next
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<A HREF="IP-Subnetworking-6.html">Previous</A>
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<A HREF="IP-Subnetworking.html#toc7">Contents</A>
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<HR>
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<H2><A NAME="s7">7. Routing</A></H2>
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<P>If you are using a Linux PC with two network interfaces to route between
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two (or more) subnets, you need to have IP Forwarding enabled in your
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kernel. Do a
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<P>
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<HR>
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<PRE>
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cat /proc/ksyms | grep ip_forward
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</PRE>
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<HR>
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<P>
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<P>You should get back something like...
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<HR>
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<PRE>
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00141364 ip_forward_Rf71ac834
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</PRE>
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<HR>
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<P>
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<P>If you do not, then you do not have IP-Forwarding enabled in your kernel
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and you need to recompile and install a new kernel.
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<P>
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<P>For the sake of this example, let us assume that you have decided to
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subnetwork you C class IP network number 192.168.1.0 into 4 subnets
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(each of 62 usable interface/host IP numbers). However, two of these
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subnets are being combined into a larger single network, giving three
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physical networks.
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<P>
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<P>These are :-
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<HR>
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<PRE>
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Network Broadcast Netmask Hosts
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192.168.1.0 192.168.1.63 255.255.255.192 62
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192.168.1.64 192.168.1.127 255.255.255.192 62
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192.168.1.128 192.168.1.255 255.255.255.128 124 (see note)
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</PRE>
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<HR>
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<P>
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<P>Note: the reason the last network has only 124 usable network addresses
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(not 126 as would be expected from the network mask) is that it is
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really a 'super net' of two subnetworks. Hosts on the other two networks
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will interpret 192.168.1.192 as the <EM>network</EM> address of the 'non-existent'
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subnetwork. Similarly, they will interpret 192.168.1.191
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as the broadcast address of the 'non-existent' subnetwork.
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<P>
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<P>So, if you use 192.168.1.191 or 192 as host addresses on the third
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network, then machines on the two smaller networks will not be able to
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communicate with them.
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<P>
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<P>This illustrates an important point with subnetworks - the usable
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addresses are determined by the <B>SMALLEST</B> subnetwork in that address
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space.
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<P>
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<H2><A NAME="ss7.1">7.1 The routing tables</A>
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</H2>
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<P>Let us assume that a computer running Linux is acting as a router for
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this network. It will have three network interfaces to the local LANs
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and possibly a fourth interface to the Internet (which would be its
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default route.
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<P>
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<P>Let us assume that the Linux computer uses the lowest available IP
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address in each subnetwork on its interface to that network. It would
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configure its network interfaces as
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<P>
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<HR>
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<PRE>
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Interface IP Address Netmask
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eth0 192.168.1.1 255.255.255.192
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eth1 192.168.1.65 255.255.255.192
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eth2 192.168.1.129 255.255.255.128
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</PRE>
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<HR>
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<P>
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<P>The routing it would establish would be
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<P>
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<HR>
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<PRE>
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Destination Gateway Genmask Iface
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192.168.1.0 0.0.0.0 255.255.255.192 eth0
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192.168.1.64 0.0.0.0 255.255.255.192 eth1
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192.168.1.128 0.0.0.0 255.255.255.128 eth2
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</PRE>
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<HR>
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<P>
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<P>On each of the subnetworks, the hosts would be configured with their own
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IP number and net mask (appropriate for the particular network). Each host
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would declare the Linux PC as its gateway/router, specifying the Linux
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PCs IP address for its interface on to that particular network.
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<P>
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<P>
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<P>Robert Hart
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Melbourne, Australia March 1997.
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<P>
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<HR>
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Next
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<A HREF="IP-Subnetworking-6.html">Previous</A>
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<A HREF="IP-Subnetworking.html#toc7">Contents</A>
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