<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN"> <!--Converted with LaTeX2HTML 96.1-c (Feb 29, 1996) by Nikos Drakos (nikos@cbl.leeds.ac.uk), CBLU, University of Leeds --> <HTML> <HEAD> <TITLE>Gateways</TITLE> </HEAD> <BODY LANG="EN"> <A HREF="node1.html"><IMG WIDTH=65 HEIGHT=24 ALIGN=BOTTOM ALT="contents" SRC="contents_motif.gif"></A> <BR> <B> Next:</B> <A HREF="node31.html">The Routing Table</A> <B>Up:</B> <A HREF="node27.html">IP Routing</A> <B> Previous:</B> <A HREF="node29.html">Subnetworks</A> <BR> <P> <H2><A NAME="SECTION004430000">Gateways</A></H2> <A NAME="tcpiproutinggateway"></A> Subnetting is not only an organizational benefit, it is frequently a natural consequence of hardware boundaries. The viewpoint of a host on a given physical network, such as an Ethernet, is a very limited one: the only hosts it is able to talk to directly are those of the network it is on. All other hosts can be accessed only through so-called <em>gateways</em>. A gateway is a host that is connected to two or more physical networks simultaneously and is configured to switch packets between them. <P> For IP to be able to easily recognize if a host is on a local physical network, different physical networks have to belong to different IP-networks. For example the network number 149.76.4.0 is reserved for hosts on the mathematics LAN. When sending a datagram to quark, the network software on erdos immediately sees from the IP-address, 149.76.12.4, that the destination host is on a different physical network, and therefore can be reached only through a gateway (sophus by default). <P> sophus itself is connected to two distinct subnets: the Mathematics Department, and the campus backbone. It accesses each through a different interface, eth0 and fddi0, respectively. Now, what IP-address do we assign it? Should we give it one on subnet 149.76.1.0, or on 149.76.4.0? <P> The answer is: both. When talking to a host on the Maths LAN, sophus should use an IP-address of 149.76.4.1, and when talking to a host on the backbone, it should use 149.76.1.4. <P> Thus, a gateway is assigned one IP-address per network it is on. These addresses--- along with the corresponding netmask--- are tied to the interface the subnet is accessed through. Thus, the mapping of interfaces and addresses for sophus would look like this: <P> <pre> ---------------------------------------- +-------+-------------+----------------+ |iface | address | netmask | +-------+-------------+----------------+ +-------+-------------+----------------+ |eth0 | 149.76.4.1 | 255.255.255.0 | |fddi0 | 149.76.1.4 | 255.255.255.0 | |lo | 127.0.0.1 | 255.0.0.0 | +-------+-------------+----------------+ +-------+-------------+----------------+ The last entry describes the loopback interface lo, which was introduced above. </pre> <P> Figure-<A HREF="node30.html#introfigip"><IMG ALIGN=BOTTOM ALT="gif" SRC="cross_ref_motif.gif"></A> shows a part of the network topology at Groucho Marx University (GMU). Hosts that are on two subnets at the same time are shown with both addresses. <P> <STRONG>Figure:</STRONG> <A NAME="introfigip"></A> A part of the net topology at Groucho Marx Univ. <BR> <P> <P> Generally, you can ignore the subtle difference between attaching an address to a host or its interface. For hosts that are on one network only, like erdos, you would generally refer of the host as having this-and-that IP-address although strictly speaking, it's the Ethernet interface that has this IP-address. However, this distinction is only really important when you refer to a gateway. <P> <A NAME="1139"></A> <P> <HR><A HREF="node1.html"><IMG WIDTH=65 HEIGHT=24 ALIGN=BOTTOM ALT="contents" SRC="contents_motif.gif"></A> <BR> <B> Next:</B> <A HREF="node31.html">The Routing Table</A> <B>Up:</B> <A HREF="node27.html">IP Routing</A> <B> Previous:</B> <A HREF="node29.html">Subnetworks</A> <P><ADDRESS> <I>Andrew Anderson <BR> Thu Mar 7 23:22:06 EST 1996</I> </ADDRESS> </BODY> </HTML>